Welcome, OP! While I'm sure you are not the only person with this issue, I think most people looking for a stroller for a child of that size are doing so because the child has medical issues that impede his/her mobility -- and as dreamfinder suggested, unless you already own a specialized stroller large enough to accommodate your little one, a wheelchair is probably your only option if you are looking for an onsite rental. If you Google "pediatric wheelchair rental" and "Disney World" you'll see there's at least one local company that offers them.
https://www.amusementparkrentals.co...tric-kids-wheelchair-rental?variant=803085929
If your child does not have mobility issues and is reasonably physically fit, you might consider foregoing a stroller/chair rental altogether -- at least until you see how he/she does on their own.