Rain boots are a unique monster. Because the material they're usually made with is neither stretchable or breathable, they're generally pretty roomy, even at the proper size. But what about the thick socks your little one will be wearing on the inside? Should kids' rain boots be a size too big?
For children, boot manufacturer Bisgaard wrote on their website that rain boots should not be a size too large, because of their already capacious interior. And while you probably worry about blisters happening as a result of shoes being too tight, rain boots are different: If they're too big, kids' feet will slip around inside of the boot, causing blisters to form. Bisgaard wrote that the boots should be just large enough that they're able to be worn with a thick pair of socks to prevent slippage.
As far as materials go, Outdoor Gear Lab reported that most rain boots are made with a natural rubber or padded neoprene. These materials make for durable, easy to pull-on boots. However, lower-priced rain boots are often a polymer blend that does not slip on as easily, so it's going to require a fair amount of trial-and-error to find the right size. It might be best to have your child wear a thick pair of socks to the store to try them on.
Should kids' rain boots be a size too large if they're the plasticky sort? Again, it depends upon the fit, but you must take care to make sure your child's foot isn't slipping around inside of the boot. The brilliant thing about rain boots for kids is that even the better brands aren't obscenely expensive, and they're pretty well-represented on sites like Poshmark and eBay, so it might behoove you to find a quality pair. My son's first pair of Hunter boots were from a secondhand store in Brooklyn. They were $15. After I cleaned them thoroughly, they were perfect, and he loved splashing about in them.
Also, stores like LL Bean and REI have experts on hand that can assist you in getting the right fit for you and your kids. Both places run frequent sales, and are usually at outlet malls as well. Having your child professionally fitted can make all of the difference in the world when you're buying something so tricky.
Each boot will be different, and it can be a real pain, but for the comfort of your kids, it's worth it.