hum, most vehicles I would recommend to someone for their kid will not run on a 4s. That is a lot of power/speed.
and most any brushed RTR truck will not handle a 3s lipo.
That would narrow it down to a brushless RTR 1/10 scale truck. But again, it will be a pretty fast truck on that battery!
Given the situation, I would say the team associated SC10rs. Why? first it is a great truck, can take a beating and handles well. Second, the radio has a throttle end point trim. This means you can run the 3s but set the top speed to what you feel the kids can handle....as little as 10mph. I have a sc10rs that I run on 2s lipos (still good for 35mph) and I simply turn down the throttle end point all the way and give the remote to my 4 year old boy. And as he gets better, I can dial up the speed a bit.
And, if you limit the speed this way, you will get a pretty decent runtime.
still, if it were me, I would get something like the sc10rs but I would sell my 4s and 3s and then buy a couple of 2s 5000mah packs (can be had for $30) You should get enough just from the 4s to buy a new 2s pack.
While the 3s could work in the sc10rs, it is kind of a low mah battery to use in a 1/10. I prefer 4000-5000mah packs.
A 2s 5000mah pack in the sc10rs with the throttle turned down to say 50% will get a good 40-50 minutes of runtime...maybe more.