This is the best response.
To add to it, I suggest OP create their own as a family. Ask the kids if there is any monster they would like to meet and then find out if they want to fight it, make friends with it, play a character as one, or whatever else.
Op perhaps could pick up any D&D campaign setting, read through it, see what it entails and try to make their own setting. This easily goes along well with the parent comment suggestion for dice, cards etc. It can be about things that are close to what is known or whatever the kids come up with, the possibilities truly are endless.
One of my favorite quotes from Gary Gygax, creator of D&D, is "The secret we should never let the gamemasters know is that they don't need any rules."