Spider bite or something else. Maybe flea bite. What should I do?

Topical ointment. For flea bites use topical ointments that help with itching like Benadryl (diphenhydramine) ointment, calamine lotions. Usually they get better in 2-7 days. If the itching is not better with topical ointments then take some Benadryl (diphenhydramine) by mouth. For kids the dose is based on weight so consult with your doctor. Occasionally bites can get infected with bacteria. Look out for lots of swelling, pain, and possibly pus.
Flea bite. Use antiseptic soap to wash fleaor spider bites. Do not scratch flea bites. You can use 1% Hydrocortisone on the bites or use antihistamines for itching. A paste made of @ parts baking soda and 1 part water can be applied to the bites.However the bigger issue is that you must get rid of the fleas. This can be very difficult so many people elect to hire pest control professionals.