My stepsons and 7 and 4. I'm looking after them today as I do most Friday's in the holidays. I generally leave them to play until they start to fight them take them to the park. But aside from a low hum of activity, they are quiet.
AIBU to just leave them and imagine they're playing nicely? Or are they shaving one of the cats/redecorating the house with toothpaste/unwrapping all the Christmas presents hidden in the baby's room?
When DS and his cousins go quiet I usually find there is an urgent task I need to do, that involves loitering just outside whatever room they are in. I can usually work out what they are up to fairly easily. You are a brave woman to leave them to it!!