It will pass. I find a dose of diphenhydramine (benadryl) can sometimes help if there is some associated itching. Most toddlers won't let you do much. If you just monitor the area for signs of infection (unusual) it will pass in the next day or two. You'll get a few concerned looks from strangers but that is the worst of it.
Cool Compresses. Cool compresses can be helpful to reduce localized swelling. Also, keeping the head elevated my help.
Insect bite. Swelling due to insect bites can be due to inflammation from the bite itself or a subsequent skin infection. If it is mild, try cool compresses. If worsening, see a doctor since an infection may require systemic antibiotics.