For instance, Player 1 has Dragonmaster Outcast on their battlefield, which has the ability: At the beginning of your upkeep, if you control six or more lands, create a 5/5 red Dragon creature token with flying. If Player 1 fails to create the token creature during their turn, can they create it during the Player 2's turn (and block an attacker with it), or create two token creatures on their next turn? I love playing this game, but the fights my 11-year-old son and I get into sometimes are nuts!
In a competitive situation, this is a "missed trigger"; as soon as you realise it has happened, you should call for a judge who will resolve the situation - what they will do well depend on the level of play: the resolution in low-level play (e.g. FNM at your local store) will likely be different from that at a Pro Tour, where both players could be subject to penalties.
In a casual game among your own family, do whatever gives you the most fun.