my DD1 broke her leg, about 3wks before she turned 3.
she had been helping in the garden.....we had just moved in and DH was laying a patio and steps, she held the patio slab as DH let go, and it fell on her leg and foot breaking 3 bones. DH felt soooooooooooo guilty, especially as he had been holding the slab and was stood right next to her, but guess what.....it was an accident.
accidents happen, and with small children that actually happen with someone very close by to them, and also in the care of someone who loves them. BUT, unless meant (in the was of a beating), it is no-ones fault......blame and guilt are part of being a parent but they are pretty useless emotions (in that we waste time feeling them and they stop us from seeing the bigger picture on how to kepp them positive etc).
now, her tears may also be from pain.....it can hurt having a cast, they are heavy (even the fibre-glass ones) and unweildly (sp?) and on the leg especially (and for an active toddler it is more frustrating), it stops you moving and doing stuff in the normal way.
with my DD1, it took 4wks before she even put any weight on her leg, and I had to go buy a double pushchair again (just as I had got rid of ours and opted fora buggy and step), and she then stayed in that double until she was almost 4....it took her that long to get her confidence back again.
I would give her lots of TLC(not for one minute suggesting that you aren;t giving her TLC tho), and lots of praise for any thing she achieves on her own. lots of calpol too........I knwo from my own breaks that the throbbing can take 2wks to settle), and listen to ehr if she says it feels odd. she may not have the language yet to describe pins and needles properly, and they are the one thing you do need to look out for, as it may mean swelling or that the cast has shifted.
good luck with all this, and please, don;t feel bad. it happens!!