2T/3T is the smallest they make. But they are smaller than 2T pants. DS could wear them when he was still in 18 mo pants. And I have found the Disney brand seems to run a little smaller than Hanes and Fruit of the Loom too, so you might want to try those. At Walmart & Target they are in the baby section where the socks & stuff are.
Maddie's are all from the baby section at Target. The 2T/3T size really shrink up after a few washes and they definitely fit her fine. I realized the other day that I must have accidentally bought the 4T size in her Disney Princess underwear. They look a little funny on (like she is wearing granny panties, lol), but they don't fall down or anything. And her 2T pants are generally a little big in the waist still.
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Re: Where to buy
Same here. And they seem to fit my little peanut okay.
Ditto.
Ditto what everyone else says Disney underwear by Handcraft.
These are the ones we used to get C. They run the smallest:
https://www.amazon.com/Underwear-Little-Mickey-Mouse-Handcraft/dp/B004LER9UA/ref=sr_1_12?ie=UTF8&s=apparel&qid=1300224868&sr=8-12
The 7 pack is the one with the smallest underwear--the 3 packs all seem to run big.