In reply to crooner:
I'd say probably something as slick as possible. ('Slick' is the opposite of 'grabby' in a belay device - what I mean by that is that it offers less friction.) Not necessarily what you want with a climber of your own weight and a relatively skinny rope, but it you seem to be experiencing a bit too much friction with your grandson.
An original Black Diamond 'ATC' is what I'd suggest. Someone above mentioned the DMM 'Bug' - I use one of those now, which I bought to replace my ATC when I bought new ropes and found it a bit too slick with them. It's significantly more grabby (a good thing, in my case, and why I bought it).
Someone else above mentioned the BD 'ATC-XP' and 'ATC-Guide' - both are newer variants of the original 'ATC' designed to add a bit more friction, or quite a lot more friction, depending on which way round you use them. The 'ATC' is almost always just known by its initials, by the way, they have effectively become the name of the thing but they originally stood for the name "Air Traffic Controller'.
If you can borrow one, maybe give it a try? If not, I'm pretty sure I have one kicking about somewhere (unused for a long time now!) - if you'd like to have a go with that drop me an email with a postal address and I'll bung it in a jiffy bag and post it to you sometime in the next few days..
It's tempting to be mischievous and suggest you ditch the harness, get some good gloves and give the kid a body belay instead. (I would, if I was certain you'd realise I was joking - it's not an entirely terrible idea, but the climbing centre would be horrified and probably ask you to leave!)