How important is a 3D hood?

I don't think there will be much difference in use IF you make sure your flat hood is large enough so that when drawn up it covers the head well while leaving the face opening where it should be. a lot of flat hoods, when drawn up, give you a nice hole right on the forehead.

to each his own – but I think I would consider the factors i mention above.

if its intended for mild weather (above 40~45) then maybe flat=cheaper+easy to use

if however you want them to go down to freezing etc i would really think about how disciplined the kids are – from my own experience (girls aged 9,7,5) that is a big issue while they try to fall asleep and during the night. Unlike adults (and experienced campers) they lack the discipline to button down the hatch and stay still enough and/or adjust after they toss around. In that sense a contoured hood that snugs around their faces could help protect them (unbeknownst to them of course

M

I personally think that in your case no hood would probably be your best option.
Much easier to make
More flexible
Lighter

I prefer a hoodless bag/quilt as I usually wear a hooded clothing item and or cap to bed, either a down sweater/jacket/vest on cold nights or a knit cap or hooded fleece on cool nights.

I would think this is a better choice for children because they will want to wear the clothing before they go to bed anyway and then all you have to do is have them cinch the top of the bag up after putting them to bed. No fiddling with the hood adjustments and unhappy kids because of the claustrophobia that can occur if they feel too constrained.

i missed that somehow…wow – i wish i had an uncle like you ;)

I made a sleeping bag for my wife and went with a flat hood. Mostly because I'd never made a sleeping bag before and wanted it to be as simple as possible. Also because she tends to sleep on her stomach with her arms under her head, a 3D hood would have been constricting. I did make the hood fairly large so that if she cinches it up it will enclose her head better but so far she hasn't used it in temps where a hood is all that necessary (nor do I think she ever will, which is why I made the bag in the first place rather than just buying one).

The biggest lesson I learned from the whole process is that I need to do a better job selling my wife on the idea of a quilt because I'm no good at sewing zippers, especially when they're five feet long.

Adam