I have the same problem, we are ECing MJ and so she wears knickers during the day when at home... as she is only 12mths there is no way that she is able to fit the 2-3 undies.

Mum found that pumpkin patch do a 1-2 size and these fit her OK - she is quite small so i am sure they should fit an 18mth old.
they are $19.95 for a pack of 5

HTH