find something very important to do whilst dd2 is in ballet. (go next door to the supermarket, whatever) turn up just in time to slide her shoes on and collect dd2.
failing that, there is a chance that her tap shoes really are uncomfortable... 
they don't have to be. i recently spent a good hour with dd1 trying on tap shoes at a local stockist and found it a little like goldilocks and the three bears. dd1 eventually settled on a particular style, but one size was too small, and one was too big, and the manufacturer was out of stock of the middle size. so we called every single dance stockist in the city, and two weeks later, have found a pair.
you do have my sympathy though, dd2 has just decided that she is giving up tap, after begging to do it (much to our fear that it would be impossible for her).
orrrrrrrrr.... take her nintendo ds and let her play it until just before her lesson starts (wait in the car if the mere environs of the dance school triggers a meltdown) and same again, shoes on just before they line up to go in.
it is likely that it has just become a habit now, tbh, and you need a new one so that it doesn't ruin her fun. ds usually works to avoid meltdowns here (unless removing it causes a meltdown lol)
but either way, distract or busy elsewhere until it's time to line up, then shoes on.