If your child doesn't have enough money in their Spend pot, they can remove money allocated to a goal back to Spend.
To do this, they will need to:
Navigate to their 'Goals'
click on the goal they'd like to remove money from
Tap on the 'Transfer' icon at the bottom of the screen
From there, a pop up will appear which shows how much money is in the goal and they can use the sliding bar to choose how much they'd like to remove
Alternatively, they can also use the minus (-) sign to get a more exact amount