I’ve heard good things about the RPS Hawk and X-Pro Hawk DLX, especially for riders who are transitioning to more powerful bikes. The fuel injection on the X-Pro is a nice feature, but the counterbalanced frame on the RPS Hawk-X is definitely a good thing for smoother rides and less vibration, which is especially helpful for beginners learning to shift gears.
As for size, at 5'8"" and 5'10"", they should fit well on these bikes. I’ve seen a lot of kids and teens around that height have a good experience with them. I’d suggest getting them a chance to sit on a few models, if possible, to see how they feel. Also, for the price point, check out
https://eridepross.us/ – I recently tried the 2024 E Ride Pro SS, and it offers some seriously smooth power for off-roading, maybe something to look into when they're ready for the next step!