3/8ths or 1/2 inch fireboxes will take longer to heat soak (thermal capacity) but, they will eventually heat soak to a high temperature if you cook long enough. For most of us, a 3/8ths or 1/2 inch firebox will work out fine for our cooks. If I was a hard core competition smoker or running a semi-commercial unit, then a semi-insulated firebox starts to make more sense to me.
Regarding kids and burns in general, even with an insulated firebox, the first and main problem is user error! Having a bit extra in terms of safety though is a generally a good idea in most applications though because no one is perfect.
Today, I think there is a strong argument for 1/2 inch fireboxes for backyard smokers. However, if I ever do a trailer mounted propane tank smoker, I'd be hard pressed to not do a semi-insulated firebox with a larger propane tank section covering a smaller one.