Instructions are on the back of the gift card.
Redeeming a gift code or gift card is a matter of a few quick steps.
You may be asked for a verification of your card.
After this, you can start adjusting the child's account limitations to your liking.Figure 4.3 The Mojang account registration pageĮnter your email address and password.If not, click 'The person I want to add doesn’t have an email address', and set up a new Microsoft account. If your child already has a Microsoft account, click through the remainder of this dialogue.Enter the email address of their Microsoft Account below if you have it.Select 'Family & Other People' from the left-hand menu.Open Windows settings and select 'Accounts'.Here's a guide to do so - I copied the instructions verbatim: If they already have an account, you're going to need access to it to set the parental control.
In your case, it's easier to initially set up your child's account on Windows 10 through your own account, after which you can set parental control on theirs, and more easily buy them products. During your purchase, you can enter the email address or 'gamertag' of the recipient, and after the payment, they will receive a link to accept the game and add it to their account.