You can use the ' Transfer' function found by tapping the quick actions icon to move money from Save to Spend. 

Use the keypad to enter the amount, and hit 'Transfer to Spend'. 

You can also switch the direction of the arrow by clicking on it to move money in the opposite direction (e.g. Spend to Save).

PLEASE NOTE: If you have locked your Child's Save pot, they cannot make a removal themselves.