One of my security settings is preventing me from accessing Forge and Custom Games in Halo 5

If your account is set as Child or Teenager in Privacy, Family settings you will not be able to access Halo 5's Forge or Custom Games. An XBOX Live account must be Adult to access that content.

Halo 5 of course due to custom forge maps.

There IS a forge world map. I forgot the name of it but you can only access it through Forge or custom games. But there is a forge world in H4. Took me a while to find it because it does not say "forge world". But it is indeed the forge world for H4.
It has an insane amount of items to pick from. Way more than any of the other maps in Forge.

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I hope they add a community playlist like they did in Reach. It allowed people to use maps from H1, H2, and H3 all in Reach for the community to play. They already know our favorite maps from existing Halo games, now we just need some sort of custom
playlist or community playlist to check out those maps. This is only the beginning, 343 will make a lot of changes just like they did with the Halo Reach multiplayer.

Halo 5 Forge is quite different from the Halo 5 game that is on console. There has been no talk on if Halo 5 will come to PC or be a play anywhere title.

Hiya!!

First off, if your 5yo is able to purchase digital content from XBOX, you should be setting up your security procedures to prevent that from happening.
You would go to setting>account>sign-in, security & passkey. Select sign-in & security preferences> select Ask for my passkey> from there you can select a custom passkey.
Next, you can remove any game from your library to prevent further gameplay.

Happy Gaming!!