We are taking the kids to London for half-term and are staying in an apartment in Islington. They really want to go to Harry Potter Studios but left it a bit late in booking and could only get a slot for 9.30am. Just a bit worried about getting there in time. I know we need to get a train from Euston to Watford Junction. In terms of buying tickets, do children need a ticket for overland trains (I know that they can travel under our oyster card on the tube) and would it be better value to use oyster cards to cover the overland train journey or buy separate tickets from National Rail?