Looks like it could be an allergic reaction to mosquito bites, quite common here, especially after school outdoor time. Try a cold compress first to bring down the swelling, then dab on some calamine lotion; my friend's son had something similar and the doctor actually recommended keeping the area moisturised since the skin gets dry and hard after the bite reaction. If the bumps are in a cluster or a line pattern, do check his mattress and schoolbag too, as that can sometimes point to bed bugs instead. Either way, since it's your child's skin, best to get a doctor to take a proper look and confirm what it is.