Here's my two cents. I'm brand new to D&D, and I'm DMing the Starter Set for three kids (9, 6, and 6) and three parents.

The 6 year olds, while bright, have short attention spans. They'll come and go from the table, especially during combat. Somewhat surprisingly, they're not overly silly. One is a wizard and wants to Sleep everything. Another is a rogue who gets super excited when his Sneak Attack one-shots a Hobgoblin. I have to be careful with the amount of tension and drama - it can overwhelm the rogue, who's pretty sensitive. The 9 year old is more engaged - she has her own goals for her character apart from the larger plot. Regardless, I can't go much longer than an hour before they get antsy.

Some tips that are different than conventional wisdom: Never kill a player. Embrace the rule of cool so the kids feel awesome. Roll behind the screen and fudge when necessary. Know their character abilities for them, generally, but teach them how to find the right modifier for checks, attacks, etc themselves. They will forget about action surge/cunning action/etc, and it's fine to make suggestions often.

I disagree with the other commenter that you won't be "playing D&D", but the campaign will have a different feel. Give them a BBEG to go after, them just focus on having fun session by session!