Allow my child to play with brother with his own account

Have you looked into the settings on your eldest's Xbox? you should be able to find a way to lighten the security so that he can access online games?? I have had a similar issue when my son got his own Xbox, you just have to adjust your privacy settings
slightly, hopefully this helps a little

it would be best to have your own separate email and not use your brother's

if you are adding the alias so your brother can use your Xbox account to play your games and/or use your Live Gold/Gamepass then it would be better for him to create his own Xbox account and for you to make the Xbox your Home Xbox, which will allow him to
play your games on his account.

the following link has all the info on setting a Home Xbox

https://beta.support.xbox.com/help/hardware-network/console/my-home-xbox

He's not transferring them to his brother. He's transferring a console license to his brother's console so his brother can access them on his own account. When you purchase digital content, you're given 2 licenses; one for your account and one for your console. The account license lets you access the content you purchased on any console as long as you're signed into your account. The console license allows others to access your games on the console with that license.

So if your older son sets your younger son's console as his Home, your older son an continue playing his games while he's signed in on his own console. Your younger son would then also be able to play the games on his own console because it has your older son's licenses.

I made the mistake of making my account the child instead of my siblings who is actually younger then me this wasnt a problem until backwards compatibility i was over the age of 18 when i got the xbox one and my brother was only 15 he was supposed to be
the child but i made a mix up at the begining is there anyway i could turn this back around and make me the adult as it would make it easier to purchase games as my brother got his own xbox now and i cant really use his account to purchase games for me

Your settings are probably on too strict - as my little brother was able to play with a child account on BF3 Beta and he's 10. Also, isn't 18 the age when your allowed to promote your account to adult? So you dont have to deal with this stuff..