The promise you make to a child belongs on your calendar, even when it sounds small.

Children keep the ledger when adults do not. A vague weekend plan, the trip to the shop, the game you said you would play later: to you these are casual sentences, and to them they are scheduled events they may have been thinking about for days. Forgetting is never read as busy; it is read as what you say does not always happen. Enough repetitions and they stop expecting, which looks like maturity and is actually a lowered estimate of you.

Write it down at the moment you say it, in the same place you keep everything else. If it cannot happen, say so early and offer a real replacement.