D&D 5E D&D 5e suggestions for two beginner children and a first time DM dad?

[humble minion]

2Kings Games has a line of products specifically intended for younger kids and their parents, designed to be playable in short 'chapters' before bedtime.

Little Feet — 2 Kings Games

www.2kingsgames.com

 

[The Green Hermit]

Character building is a major part of D&D but it will unavoidably stretch the time from when you manage to have everybody sitting down at the table to actually starting the adventure. If you don't have especially quiet and attentive kids, chances are it will become a mess while you try to guide all of them through it at once. If you want to absolutely do it, I'd rather take aside one kid at a time maybe some day BEFORE playing, and just build the character, so that everyone is ready on gaming day.

That's what I do. The second time around, I ask them at each part, but have them do everything they can on their own. The third time, they are on their own and ask help when needed.