OK, you need an arrangement that will last for 7 years until the child is old enough to claim the money.
If the executors are willing to look after the money for the 7 years, they can set up an account for the purpose in their joint names, documenting (e.g. in a letter or note) that it used solely for the purpose of holding the child's bequest until the child is old enough to claim it. This will not affecgt the personal tax affairs of either executor.
However if the the sum is large a question arises as to whether simply putting the money in the bank for seven years is the best way of handling the matter, in the child's best interest. The executors should ask the solicitor what courses of action are open to them under the will, and then ask themselves — and if they think it desirable, ask an accountant or investment adviser — which course of action they should follow.