How can I stop/block my children’s accounts on Minecraft pe from joining servers of people...

It depends on how the accounts are actually setup to begin with. Are these part of a Family or are they individual accounts? Under family settings, you can make use of these resources to set various limits.

https://www.xbox.com/en-HK/community/for-everyone/responsible-gaming

They have their own gamer tags (associated with their own email addresses) BUT are part of a 'family' setup where I am the owner (as per Microsoft profile).

I have read through the community page but still can't see where I can set a limit on my son joining a 'featured server' where there are people that we don't know.

From Minecraft/Microsoft -

https://minecraft.net/en-us/article/minecraft-multiplayer-server-safety

“Join Multiplayer Games” Settings
If you have your settings set to block joining multiplayer games, you can’t join any multiplayer games in Minecraft, including shared local worlds, Realms or servers. To enable multiplayer in Minecraft, make sure this setting is set to ‘Allow.’

However, just like ASCllnaut, that setting was already turned on for my children. In other words I'm in the same boat, we can play on the LAN but not on a realm.

You can join realms in mincraft pe, but my friend can't join in any server for some reason.

I need you to change my minecraft pe so then i can join to other ppl or friends