Our 8 year old makes good use of the Rival Hera, despite the massive grip. He's also not afraid to be hit by them.
The hardest part for our 5/8 year olds is loading mags and not releasing rev triggers between shots.
The best blasters I've found for their frustration free foam warfare are:
A. For kids who have a hard time loading/using mags, the Fearless fire, or the Incisor (which is a little bulky). Honorable mention goes to the barricade as well, but it should be said that it requires significant modification in the form of a rev trigger and new (worker white or insutanto) flywheels to be up to snuff.
B. For kids who have a hard time with the rev trigger, the Buzz Bee Brute is a no brainer... It's our five year olds go to blaster. Again honorable mention to the Barricade as a 10-shot pistol with an on off switch rather than a rev trigger (though I find the jam-door lock to be a little more convenient to cut power to the motors) alas it should be properly modified in both cases.
C. You can't really go wrong with a front loader, either. Personal faves are the Sweet Revenge/Hammershot, Rough Cut, and Messenger/Luminate.