With orders of $25 or more, Amazon offers the Oral-B Kids' Pro-Health Disney Princess Power Toothbrush, model no. DB3010, for $3.74. Clip the coupon on the product page to cut that to $2.74. Plus, Prime members bag free shipping. (Non-members can spend $49 or more to qualify for free shipping.) That's the lowest total price we could find by $2. It's recommended for kids aged 5 to 7, and features extra-soft bristles. It requires one AA battery (included).
Note that this item is part of Amazon's "Add-on program" and can only be purchased with an order of $25 or more.