Curfew – Is walking on the street in Tokyo legal after 11.00 pm if you're under 18?

In reality, as long as the boy is accompanied by an adult, just walking on public streets in Japan late at night won’t cause a substantial problem in most cases. If you are unlucky, a police officer might stop you and ask a few questions, but anything worse is unlikely to happen. Entering certain safe stores like convenience stores is OK, too.

But I don’t recommend taking your child out to certain red-light districts like Roppongi or Shinjuku. It can be a risky thing, both physically and legally. I’m not really good at legal talks, but according to this Japanese lawyer, I think the interpretation by @fkraiem is correct:

保護者同伴だったとしても、例えば、深夜遅くまで歓楽街や風俗街を連れて回るなどの行為に及んでいる場合には、やはり規制対象となることでしょう。保護者同伴であれば、なんでも許されるというわけではありません。
(My translation: Even when accompanied by a guardian, for example, if you took a youth to a nightlife/red-light district, that would be a subject to confines. Accompaniment of the guardians does not mean everything is allowed.)

If you violate this legislation in question, theoretically, you might be fined up to 300,000 yen. (Because your son is under 16. Oddly, there is a written penalty regarding youths under 16 (not 18). See 第26条第5項.) But I guess the worst thing that can actually happen in your case is that the police officer tells you to return to your hotel with your son immediately.

And many kinds of shops and facilities deny admission of people under 18 after 11 PM, even when accompanied by an adult. That’s a regulation imposed on the shop’s side. Such shops/facilities include restaurants serving alcohols (aka izakaya), movie theaters, karaoke boxes, amusement arcades.

As stated in fkraiem’s answer, the relevant legislation applies only in Tokyo (although quite a few other places have similar laws). Here’s the section in question:

東京都青少年の健全な育成に関する条例

青少年　十八歳未満の者をいう。

十八歳未満の者をいう

(深夜外出の制限)

第十五条の四

保護者は、通勤又は通学その他正当な理由がある場合を除き、深夜(午後十一時から翌日午前四時までの時間をいう。以下同じ。)に青少年を外出させないように努めなければならない。

2　何人も、保護者の委託を受け、又は同意を得た場合その他正当な理由がある場合を除き、深夜に青少年を連れ出し、同伴し、又はとどめてはならない。

3　何人も、深夜に外出している青少年に対しては、その保護及び善導に努めなければならない。ただし、青少年が保護者から深夜外出の承諾を得ていることが明らかである場合は、この限りでない。

4　深夜に営業を営む事業者及びその代理人、使用人その他の従業者は、当該時間帯に、当該営業に係る施設内及び敷地内にいる青少年に対し、帰宅を促すように努めなければならない。

A quick, sloppy and incomplete translation:

Tokyo Metropolitan Ordinance Regarding the Healthy Development of
Youths

A youth (minor, 青少年) is a person who is under 18.

Article 15.4: Limits on going out late at night

1. Excluding commuting to work, school or another good reason (その他正当な理由), a guardian must not allow a youth to go out (外出させる) late at night (深夜), defined as between the hours of 11pm and 4am.

2. Without the permission of the guardian, or another good reason, no-one may take a youth out (連れ出し) late at night.

3. Any person shall endeavor to guard (保護) and guide (善導) youth going out late at night, unless it is clear that they have
   permission from their guardian.

4. Staff working in establishments open late at night must endeavor to return any youth to their homes.

Now the wording is a bit ambiguous here, but per consultation with Japanese.SE, the two clauses in section 1 are independent and a youth thus requires both the permission of the guardian and a good reason to be out at night — even if physically accompanied by the guardian. "Sightseeing" and "shopping" would likely not qualify as good reasons, although eg. "going to the airport" certainly would.

That’s the legal theory. In practice, I really don’t see Japanese police stopping a parent out with their children at night, unless they’re in Kabukicho about to enter a pink salon or something — and the rest of the ordinance goes on to outline a whole bunch of places youth at night are never allowed, including movie theaters and virtually all forms of nightlife. Shops and city streets, though, are not on the list, and if you were stopped on the street, the statement "we’re going home/to our hotel" would in all likelihood be more than sufficient.

Updated after clarification from Japanese.SE.

The relevant legislation is here (in Japanese). This is only for Tokyo, the legislation in all other prefectures is similar but may differ in some details.

Basically, it’s “illegal” for parents to allow a minor to go out between 11pm and 4am except for going to work/school or for some other “justifiable reason”. What constitutes a “justifiable reason” is as usual unspecified and left to the judgement of police officers/judges/etc. I put “illegal” between quotes because the ordinance says you must not do it but it’s not clear what the penalty is, if any. It’s also not clear if hotels are considered “home”, so it’s possible that he will technically be out of his home anyway.

Edited to add : there is a difference in interpretation between jpatokal and I, so here is mine in more detail. jpatokal correctly translates clause 1 as saying that a parent must not allow a minor out between 11pm and 4am without a “good reason”. However, in his commentary below his translation, he adds “by themselves”. As far as I understand the Japanese text, there is no reason to add this, and clause 1 applies equally no matter by whom, if anybody, the minor is accompanied.

Clause 2 forbids anybody from accompanying a minor out without a “good reason” or parental consent. Presumably, the bar for a “good reason” is higher here, since it allows anyone to take a minor out without parental consent, maybe even despite parental oposition in extreme cases. In normal situations, however, parental consent is required, and by giving it without a good reason, parents violate clause 1.

Other edit : I will also add that the bar for a “good reason” in clause 1 is probably quite low. Yesterday at about 11:30 just after first posting this answer, I went to the convenience store to grab some late night snacks. There was a boy probably no older than 10 with his father also buying stuff, and the staff didn’t say anything. So being hungry and needing to buy food probably qualifies. (I am not in TOkyo, but the law where I live is similar.)