In our case, my grandmother always calls my parents home phone number. The phone provider only offers simple call forwarding, so we route all calls to the home phone over to a Twilio number. We have a very simple Twilio Studio flow that routes calls from my grandmother's phone number to the call handler script we created, and all other calls are routed to my dad's cell phone number: https://share.cleanshot.com/ywwhzJ8H
We are a little lucky in that we can forward calls from our landline to a cell phone. If your relative is calling you directly on your cell phone, your carrier would need to offer selective forwarding for this strategy to work
Either route all calls from that callerid to your twilio number, port your own phone number to twilio and route all other calls to your new number, or what I would do: replace my mom's phone with an ATA and setup a dialplan for how to route calls to the child's number.
Many phone providers actually offer what they call “Selective Call Forwarding”, which allow you to set up a limited number of basic routing rules to forward calls coming from specific numbers.
