Child came home from daycare with sunburn

My son is 20 months and has been in daycare since he turned 12 months. Never had any problems. We love his teachers and they’re always super diligent about changing him (he’s never once come home with a rash) and letting us know if he fell or whatever and got a scrape.

Well it’s been 80-90 degrees here the last 3 days and his class is only 1-3 year olds so they go out in the morning only for about 20-30 mins.

He came home today and his forearms on both arms are beet red and sunburnt. I didn’t even notice it when I picked him up until we got home.

I feel bad to say anything since they’re so good about everything and adore him. Should I just send him with sunscreen on?

They often go out for a bit in the afternoon too so I’m just worried it would’ve worn off by then anyway