It's best to create a new Lifecake for your child and then to delete the old one.
To delete it:
Step 1 : Once you are signed into your account on the Lifecake Website, click on the child's name (or, if you have multiple children as seen below, on the number of the children) at the top left corner.
Step 2: Click ''Edit'' next to ''Your children'' (circled in red below):
Step 3: Click on your child's name and then click on ''Delete child''
Step 4: Confirm removal by clicking ''Delete''