score:6
Yes, you can. The two adults will travel together using the two together card, and the child will travel free.
Note that even after the child turns five, you can still use the two together card, but you'd also have to buy a separate ticket for the child at full child rate.
Basically, the two people named on the card have to travel together, but there is no restriction on who else may be travelling with them on a separate ticket (or none in the case of an under five)