Will my child need to get a new phone number?

No !

As the MMGuardian Phone does not come with a phone plan, you are free to continue to use an existing airtime plan, or set up a new plan from your preferred provider.

If you already have an airtime plan for your child which you wish to continue using, just insert the SIM card into the MMGuardian Phone.

If you have an airtime plan for your child but want to switch providers, you should be able to transfer (port) the old number to the new plan. When setting up a a new airtime plan, most providers will offer to transfer an old number as part of the account set-up process.