I agree it's called "Summer Break" for a reason, but you should never stop learning. In fact, you learn every day. It doesn't hurt to maybe relax but neither does learning a thing or two. Sitting home playing video games all day for 2 months is just useless, and pointless. A waste of 2 months every year. Learning is a great experience if you embrace it. So yes, parents should make kids study during summer vacation ( just not too much ).
It turns out a 90 day break is a really bad idea for anyone doing anything. Ever take a 90 day break from working out and if you did were you surprised you're not as fit as you used to be 90 days ago? Because if you're surprised you're a fool. The brain requires the same training.
Kids need a break during summer to function well during school time. If a student does not get a sustainable break, they will not function well enough during school and it will feel tiring and unfair to the kids. If a student does not get a break they will most likely not pay attention.