Configuring the Client Software
These instructions are for Java edition - Bedrock and Pocket (mobile) editions may differ slightly but are relatively close in configuration.
Launching the Minecraft Client
When you first launch the Minecraft client, you'll be asked to log in. Log in using you child's Microsoft Live account. Once logged in, you'll see a Play button. To the left of the Play button there is a drop down menu. Make sure Latest Release is selected. Then press Play. The main menu will load in a new window. In Windows, the main menu window often loads small. If it does just maximize the window.
If you're using Bedrock, you won't see a Play button. You'll go directly to the main menu, as described below.
The Main Menu
The main menu will have show several options, including Singleplayer, Multiplayer and sometimes Microsoft Realms. You want to choose Multiplayer.