unable to buy xbox gold ultimate for £1 - child account - you need a parent to buy this

You need to add your child's account as your dependent and give him permission to buy games and subscriptions.

Have you tried adding funds to your child account?

The smallest amount you can add is £10 and details can be found here:

https://support.microsoft.com/en-gb/help/4027024/microsoft-account-add-money-to-your-childs-account

If the date of birth on your Microsoft
account shows that you're under a certain age,
Xbox is required to request
parental consent for you to use Xbox Live. If you're an adult, but you got a message asking for
parental consent, it's because you're a child in your Microsoft family.

To approve your child's purchases on Xbox One

1. Press the Xbox button to open the guide.
2. Go to System > Settings > Account > Family settings.
3. Select Manage family members and choose a child account.
4. Select Privacy & online safety > Xbox Live privacy > View details & customize > Buy & download and select On from the drop-down.

A child account and a parent account are 2 seperate accounts so you will need to buy gold on each account unless you bought a gold family pack for the parental account and then both accounts would have gold on the same plan.

You don't need to buy separate gold for each account.

You can set Child 1's Xbox as child 2's home Xbox, then only buy gold for Child 2's account.

I hope that makes sense, if not I can re-explain.