I use road bar tape. You can make the grip as thick or thin as you want, and it comes in a ton of colors from the kids to pick from. It's also easy to customize the grip width the work with small hands and grip shift. It's cheap, too.
I just picked some up for my 2 little ones. The other benefit is that since they do not have a pattern, they can be cut down. Adult grips are quite a bit longer than they need to be for little hands. They come in cool colors and can even be ordered direct using 2 or 3 colors.
I got some small thin grips for my daughters bike, and they don'thave enough cushion. She wears gloves for trail rides, and is fine. Around town though without gloves, she complains that her hands hurt. Might have to try some foam grips. I have ECI Racers Edges on my bike, but on hot days without gloves, they get slick with sweaty hands.
Another vote for the ODI Ruffians. They have a small diameter and grip well, with or without gloves. My son has been using them since he was 8 and he loves them. I have them and love them too FWIW. Another thing to consider is that with the Ruffians, he will be able to continue to use the same grip as he grows instead of changing to another grip when he gets older and needs adult sized grips.
Again with ruffians, but im going the 90mm inserts for grip shift but with locking clamps both ends. I let my son try mine while I was giving rogues a go, he liked them (and I went back to them, rogues now got moved to my commuter, great for riding without gloves)
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