An easy "swap sessions" feature is yet to be added but you're able to do a little trick that will get your register right.
Note: Just make sure that the sessions you swap are priced the same
- Set the child to be on holiday for the original session. This will remove the child from the register, but the session will still appear on the invoice
- Add the new session as an ad hoc purchase on the right day in order to make the child show up in the register. Remember to add a full discount of the ad hoc session so the parents aren't charged twice.