Reincarnation of a Sotapanna person (Stream Enterer)

There is nothing in the Pali suttas to support answering any of the questions.

For example, there are no stories of Sotapanna reappearing as human after death. Instead, every story in the suttas has Sotappana reappear as a deva (god), such as in MN 143.

There are no stories in the suttas of a child born with Supramundane Knowledge. For example, the suttas have a short verse (Thag 6.10) about a 7 year old Arahant however the sutta merely says the boy was "educated and trained" by Arahant monk Anuruddha.

Also, there are no accounts of contemporary famous (reputed enlightened) monks who claimed to have Supramundane Knowledge from a young age and did not acquire Supramundane Knowledge from study and practise.

I'm answering this with some experience I know similar to this.

It's says that 7years is the minimum age that it would take to understand Dhamma. I haven't seen a reference for this but it is said by many people. An example for this is prince Rahula become a monk when he was at age of 7years.

Anyway according to some stories I've heard present day, They no need to practice anything, they them selves starts to realise that there's a problem in the things they do. That's because they have the right view. So that right view starts to conflict with the view of the other people (puthujjana). So with their understanding they have started to search for Dhamma. And they have come and stopped at some monks. And these monks' view is not accepted by the majority of the people (same as in this forum). But those stream entrants people say that it is what they have looked for.

How long does it take?

Can't give an exact answer to that. But as mentioned earlier it's said that minimum age is 7. With the experience it can go up to 20, 30 as well.

Only way to tell about past life is through pubbe-nivās-ānussati-ñāṇā (which is used to see the past lives) which can be obtained by meditation, or by asking buddha. But a rough idea can be get by asking the right view

With metta..!

When the life of a Sotapanna ends, let's say he/she reincarnates as a human.

Yes, Sotapannas can either come back as devas or humans for seven times at most:

"Again, some person fulfills virtuous behavior but cultivates concentration and wisdom only to a moderate extent. With the utter destruction of three fetters, this person is a seventimes-at-most attainer who, after roaming and wandering on among devas and humans seven times at most, makes an end of suffering. This is the ninth person, passing away with a residue remaining, who is freed from hell, the animal realm, and the sphere of afflicted spirits; freed from the plane of misery, the bad destination, the lower world" ~~ AN 9.12 ~~

.

Does that mean, the reincarnated is capable of demonstrating the qualities of a Sotapanna since a very young age?

Quite possible. It's like the cases of those music or math prodigies who demonstrate extraordinary potential at very young age. A more concrete example related to the Dhamma was when the Buddha attained the First Jhana when He was a kid!

Or, does it take years of practice (again) to regain Sotapanna qualities ?

Since it's all about potential and capability, it's not that one has to "regain" Sotapanna qualities, but it's just that there're various differnt sub-levels within Sotapanna: some do take the maximum span of 7 lifetimes, some only takes 1 life, etc. So, again, really depending on how much one cultivated in his previous lives that he could either be like a Mozart, or a Chopin, or a Paul Mauriat.

And is there anyway to tell if a kid attained Sotapanna in his/her past life ?

It'd take a Sotapanna himself to really know AND experience firsthand all the Sotapanna's quality, in order to truly tell if someone else is also a Sotapanna. However, just like the attributes of a math prodigy, a Sotapanna kid could show early signs of Dhammic maturity, some natural instinctual tendencies toward virtues, calmness, wisdom( Sila/Samadhi/Panna ) without much coaching or close guidance from his/her parents.