Schools typically cannot apply sunscreen on a child.
This is common.
So the parent has to apply it on the child, before school.
Get the kind of sunscreen that is water resistant and the "sports" type.
That lasts longer.
Also, kids should NOT be in the sun ALL day.
They should have access to shade... and be given breaks to be IN the shade etc. Or the group leaders should be having periods where they lead the kids into shadier areas.
Being in the sun ALL day, for child or adult, is not good.
Being in heat all day, for a child or adult is not good.
So ask them: will the kids be ALSO given the option or direction, to go in the shade?
Or if a child does NOT want to be in the sun, then what??? Ask your provider that.
AND, do the kids have water bottles or access to drink water, when they want to?
For some, being in the sun/heat/hotness for even 1/2 hour, is a lot.
I know, I live in Hawaii.
And this is what schools do here.
Also, even if your son is 3... teach him to say "I am hot..." etc., to the group leaders. And teach him to say "I need water...." to the Group Leaders.
*ADDING THIS:
Does the playground or whatever, have trees, for shade? Or a building structure to go under for shade?
3 year olds are young, and some may not want to be in the sun or may want to play in the shade.
Regardless, you can ask the Provider(s) that.
If they have enough adult to kid ratios, then they can certainly observe the kids and if some kids are languishing under the sun.
Also, people can get sunburned even in the shade.
I know this personally. And also being I live here in Hawaii.
Lots of sun here.