How should I treat a plantar wart on my (4 year old) child's foot?

Depends. If your child will cooperate with a regular treatment plan you can use otc wart liquids/duct tape/cuticle trimmers to take it down below the skin level.You must get the base of the wart out or it regrows.Physicians have access to liquid n20 applicators that can freeze the wart(liquid nitrogen should not be used -nerve injury).After 1-2 treatments its gone in 1-2 wks.
Depends. There are many wart treatments, and none work all of the time. Put vitamin E oil on the wart and irritate it with a pumice stone or nail file (never use it on normal skin/nails again!) and then cover with duct tape overnight. Repeat nightly. You can also use an over the counter freeze treatment every 2 weeks (with vit e every night between freezing). If not better in 8-10 wks, see your doctor.
Several treatments. You can try over the counter wart removers, but be careful: i've seen them do more harm than good. Other options your doctor might offer include cryotherapy (freezing the wart), electrocautery (burning the wart), laser treatment, or several different topical chemical treatments. If the treatment is typically uncomfortable, your doctor can numb the area before proceeding.