I had to make the same decision recently for two kids. Because I live in PA, and the kids wanted to hunt with the guns, a percussion was not an option. I made my choice after letting them both handle several flinters that were available at local shops.
My daughter liked the Traditions PA Pellet flintlock and I like the idea that when she has a little more experience she will be able to load it herself without having to measure the powder, and further down the road she will be able to load it with loose powder.
My son chose the Traditions Deerhunter. It is a light and handy little gun with decent balance.
I got both guns with synthetic stocks. My daughter had no trouble with the flash, and my son seemed to adjust to it quickly. Both guns are fifty caliber and with loads of up to 75 0r 80 grains of 3F and 2 50 grain pyrodex pellets in the PA pellet, the kids manage the recoil just fine. In fact, my son also shoots my .58 cal. Renegade with a PRB over 90 grains of 2F without complaining (this was while wearing a heavy coat.)
I think the best thing I did was to allow them to shoot my flinters first and then allow them to take part in choosing their own guns.