Bank

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.