The account I have set for my home Xbox has game pass ultimate but unless that account is...

Only one account can have a console set as the Home console at a time. Whichever account has the Game Pass Ultimate should be the one setting the console as Home. The other accounts do not have to set up as Home on that console.

Redeem the game pass ultimate on one of the kids accounts who has either Xbox 3 or Xbox 4, and then make the other Xbox the home Xbox, using that kids account with the Home Xbox setting.

No, you cannot use your game pass ultimate on 3 Xbox consoles at the same time. For starters, you cannot log in on the same account on multiple Xbox consoles at the same time.

What you can do is share your game pass ultimate subscription on one console, by setting that console as the home Xbox. Once you have done that, then anyone using the Home Xbox can access the game pass ultimate subscription without having to log in with
the account that has the subscription.

You can still log in with the account that has the subscription and play your games and access gold as much as you want on any Xbox, and your son can use his account on the home Xbox and access the subscription on there.

For more info on the Xbox Home setting have a look here.
https://beta.support.xbox.com/help/hardware-network/console/my-home-xbox

i got a gift card for some game pass ultimate but ended up using it on the wrong account and now it has basically gone to waste so is there any possibility to have it transfered to a another microsoft account??

If your trying to transfer game pass to another account you can go to settings, personation, my home Xbox and you can set your console as your home Xbox which will allow your other account to use game pass as long as its on the same Xbox

Since her account is still set as a child account, it would not be possible for her to get access for Game pass Ultimate on her own account unless the parent account will remove the restrictions for her.

Make sure you have already also shared the subscription from your account to hers using the Home Xbox feature.

Log into your account on her Xbox and do these steps:

Press the Xbox button  to open the guide.

Select System > Settings > General > Personalization, and then select My home Xbox.

Select Make this my home Xbox to designate the console as your home Xbox.