Bank is signed by finger-spelling B-A-N-K fast, as you hold your dominant hand out in front of you, palm facing outward, as you slide your hand down a small distance and progress from 'B' all the way down to 'K'. For your toddler, the sign is too fast, so they will probably only catch the first 'B' and the last 'K'
To sign bank and not mistake it for back, just remember that bank is directionally signed up-down while back is signed front-back.
Teach your child the bank sign when you are actually going on a banking errand. Concepts relating to money are abstract and irrelevant to your baby, but you may introduce it to your toddler once they show curiosity for places and activities outside your home.
This is a useful flash card in conjunction with money and counting. It is good to teach children the value of saving money early on, as well as spending it properly.