I sympathize but have no answers. I got 2 packages of size 4 underwear from Target for my kid. One fit. The other one was way too big.
I buy on Amazon. I like to be able to read the reviews and see if people say they run big or small or just right!
We got some at target and really like them
Here they have 2-3, 3-4, then 4-5. Mines from Kmart we bought them from 2-3 size he’s now 5 and just gone up to 4-5 size.
Amazon
I picked up 18 m size on Amazon.