One way to get kids, even young kids their first taste of D&D is to do roll offs. A roll off is just when you take two characters, a d20, a d6 and a d10 (optional). All AC’s are 10 and all HP’s are 10. You take turns rolling the d20, ten or better hits, then the d6 for damage until someone runs out of HP using the d10 as an easy HP counter/tracker. The advantages to this, especially with young children, is the battles are quick and fit in with the short attention span of children, the rules are easy to remember and there are very few of them, and you can also use children’s toys as the creatures to make it more fun and add a new level of play to their already existing games of pretend (that batman action figure vs joker figure).
It’s an easy system to build onto as well so once they are comfortable with these mechanics you can start adding advantage rolls, rolling 2d20 and taking the higher of the two (batman can gain advantage by attacking joker from a high place) and modifiers (batman gets a +1 to his AC because he has a Bat-suit) one mechanic at a time so that children aren’t overwhelmed.