Kids love it, but yes young ones tend to fall asleep. Use a car seat for children -- it buckles right in as it would in a car and gives them a better view.
I always had kids sit in the back seat unless they were old enough to not play with all those cute knobs and gizmos. (Like the fuel or yoke!) Having another adult along is a good idea.
I also have them use those small foam ear protectors, as their ears are probably more sensitive to noise than an adults and they may be too small for a headset or may find it uncomfortable. If you do have headset(s) that fit, the older one at least would probably enjoy a commentary. Fly over their house or yours or their favorite park or someplace else they know.
And bring an airsick bag or two just in case, but I never had a problem with that.