4 year old said bee bit her on back of head. No prior knowledge of bee stings. Result, bump on the back of her head, 2inch in diameter. Small red dot?

Ice, ice. Try ice, benadryl (diphenhydramine). If this just happened this should calm things down. If worsening symptoms get checked by a doctor as soon as possible.
Typically just obser. You need to make sure she doesn't have a severe allergic reaction ( facial swelling, difficulty breathing, hives). If this happens, call 911. Otherwise you just treat with Benadryl (diphenhydramine) and topical hydrocortisone if it is itchy. If it starts to look infected, she also would need to be seen.