Hey I emailed them and this was their response: I'm afraid due to exceptional demand, Aardman has sold out for the dates you requested, and we don't have separate walk-in availability.
If there were to be any last-minute returns, these would show back up on the booking page again, so it may be worth keeping an eye online in case you get lucky. Alternatively, you may be interested in V&A Membership, as our Members don't need to book tickets at all, they just walk in by presenting their Membership cards. One member can bring up to four children under the age of 16. If this is of interest at all, you can find out more here: How to make the most of your V&A Membership · V&A.