Home Depot won't have the type of wing nut you will need. These are made especially for the threading found on hubs, usually 9mm x 1mm. Check these out on eBay:
https://www.ebay.com/itm/Gripfast-No...25.m3641.l6368

While a bit on the pricey side, they are well made and you can transfer them onto the next children's bike that your child moves onto. Sometimes similar wing nuts can be found at a bicycle co-op. They were more common in the early Seventies just as the bike boom was hitting. Happy hunting!