It's very rare that you'll be in the situation where your child completes a goal and you're in a position to fulfil it right away! So to avoid approving the goal and forgetting about it, you have the option to approve the goal but fulfil it later.
To do this, open the goal that needs approving, and click to Approve the goal.
You'll be taken to the following page where you'll have the option to deduct it then and there, or you can tap the button 'I'll fulfil & deduct it later'.
If you choose to fulfil & deduct the goal later, the goal will stay on your child's dashboard in a pending state, which you can go in and approve when you eventually make it to the store or purchase it online.