1I alone is sufficient while II alone is not sufficient
2II alone is sufficient while I alone is not sufficient
3Either I or II is sufficient
4Both I and II are sufficient
From II, Q being in the middle, there are 10 children to his right as well as to his left. So, Q is 11th from the left. From I, P is 15th from the left.
Thus, from both I and II, we conclude that there are 3 children between P and Q.