From Hmrc gift aid rules pasted below. Could he pay for the children's entry online? Leaving you to pay for yours?
The rules also discuss family membership in a different section. It appears he could gift aid a national trust family membership for example. You and the children could then attend.
Family membership
3.37.8 Many organisations offer family membership arrangements that give all the rights of individual membership, but at a reduced cost. Where a charity offers family membership the subscription is a gift to the charity provided the subscription:
secures membership for the donor
satisfies the conditions in ‘Membership subscriptions’
3.37.9 The fact that the subscription gives members of the donor’s family rights of membership too doesn’t change this as the payment is, primarily, a gift from the donor to the charity. The donor must, however, be the person who has given a Gift Aid declaration to the charity.
Paying other people’s subscriptions
3.37.10 The payment to a charity to secure individual membership rights for a person other than the donor aren’t gifts to the charity. This includes an individual membership purchased for a family member (spouse, parent etc) that’s not secured as part of a family membership scheme. This is because although the payment is made to the charity the gift is to the person whose membership subscription is being paid.
However, this doesn’t extend to payments made in respect of a donor’s minor children (children under 18 years of age). So, a payment that satisfies the conditions to be treated as a gift if made in respect of the donor personally will be accepted as a gift if it’s made for their minor child.