Walmart discounts a selection of toddlers' and kids' slippers, with prices starting at $2.88. Opt for in-store pickup to avoid the $5.99 shipping fee. Note that prices vary by size for many styles. A couple of best bets, both available via pickup:
REI offers the Bogs Kids' Digital Camo Rain Boots in Black or Olive for $34.73. Opt for in-store pickup to avoid the $5.99 shipping fee. That's $15 off and the lowest price we could find. They're available in sizes from little kids' 7 to big kids' 6.