With orders of $25 or more, Amazon offers the Oral-B Pro-Health Stages Power Kid's Electric Toothbrush in Disney Princess, model no. DB2010, for $3.99. Clip the $1 off coupon on the product page to cut it to $2.99. Plus, Prime members grab free shipping. (Non-members can spend $35 or more to qualify for free shipping.) That's the lowest total price we could find by $1. It's compatible with the interactive Disney MagicTimer app and uses 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.