If you don’t see the friend your child wants to connect with in the “Kids of Your Friends” list, this means the child does not have Messenger Kids yet. You need to invite their parent to download the app for the child. You can easily do this from your Facebook account, or from your child’s Messenger Kids app.

From the Facebook app:

Tap the Bookmarks icon (three small horizontal lines located in the top right corner on Android or the bottom right corner on iOS).
Scroll down and tap on “Messenger Kids”.
Tap on your child’s name/account.
Tap “Choose People.”
At the top of the screen, tap “Invite,” where it says “Can’t find one of [your child’s] friends? Invite their parent to get them the app.”
On the next screen, select the parent of the child you want your child to connect with and tap “invite” next to their name.
You will see the message it will send them with information about the app. You can add more context to the message if you want. Then press “send.”
The other parent will receive your message on their Messenger account, along with a link to sign their child up for Messenger Kids.
If they then create a Messenger Kids account for their child and approve your child as a contact, the two kids will be connected. This will not connect your child with the other child’s parent.

From Facebook on desktop:

Expand the “Explore” section on the left hand side of your screen.
Scroll down and click on “Messenger Kids.”
Click “Choose Contacts.” Then click “Invite Other Parents” on the left panel or click “Invite,” where it says “Can’t find one of [your child’s] friends? Invite their parent to get them the app.”
On the next screen, select the parent of the child you want your child to connect with and click “invite” next to their name.
You will see the message it will send them with a bit of information about the app. You can add more context to the message if you want. Then click “send.”
The other parent will receive your message on their Messenger account, along with a link to sign their child up for Messenger Kids.
If they then create a Messenger Kids account for their child and approve your child as a contact, the two kids will be connected. This will not connect your child with the other child’s parent.

From the Messenger Kids app:

In the Messenger Kids app, your child can tap the “Ask to Add a Contact” tile.
Your child then types in the name of the person they want to add and taps send. Your child does not need a username or telephone number to do this. Your child simply types a name in his or her own language so you know who they are referring to. Your child requesting a contact does not add a contact, but rather invites you to add that contact for your child.
You’ll get a message through Messenger that says “[Your child’s name] has requested for you to add [Person’s name] as a Messenger Kids contact.”
Tap on this message in your Messenger account.
Tap “Choose People.” Then tap “Invite,” where it says “Can’t find one of [your child’s] friends? Invite their parent to get them the app.”
On the next screen, select the parent of the child you want your child to connect with and click “invite” next to their name.
You will see the message it will send them with a bit of information about the app. You can add more context to the message if you want. Then click “send.”
The other parent will receive your message on their Messenger account, along with a link to sign their child up for Messenger Kids.
If they then create a Messenger Kids account for their child and approve your child as a contact, the two kids will be connected. This will not connect your child with the other child’s parent.