Motorcycle superstore and rocky mnt ATV both have kids sections which I used to buy their riding gear. For helmets, the best luck I have had was going into a shop, because all of them fit differently. Plus you can ask someone who sells helmets all day, how they think they fit. (Reassurance ) My kids all wear Polaris youth helmets. They come with growth inserts. Little peices of foam you can add or take out as their skulls form. They have the best selection, at least in my area. They gave me a discount for buying them all together.
For use in the SXS (especially in the summer) you might look at the half helmets. They would be a little cooler and being strapped in a SXS you shouldn't need a full face or open face (nets, seat belt and roll bar keep you in).
Dont forget that if your youngins are under 13 years old that their neck muscles and bones and still growing and strengthening. It would be a good idea to buy a neck brace for them while you are looking at the helmets. Heavy helmets on small necks could mean injuries!