question for chess players plse

[Theresa]

My 6 yr old ds and i are learning chess. We have the 'quick chess' game and i think we are alomost there but still playing on a small version of the board with not all pawns and also missing the 'double' pieces as well. However I have a question, when you move your piece into a position where you can capture the king on your next move, you are supposed to say 'check'. What happens if you forget, are you not allowed to capture it on your next move?

If they King's in check, the checked player cannot allow it to remain in check, and must make a move that blocks/captures the checking piece(s).

If you forget to say, it doesn't matter; they should have noticed, and taken action... as far as I remember...

Thank you