Groups and changing numbers

I recently added my children to a group account. When my first son got his sim, I activated it in the group account and it asked about transferring number, pressed link to do this, followed instructions and it transferred his number to mine and I lost mine. After lots of faffing I have eventually got my number back and my son has his own number again. 
My second sons sim hasn’t been activated yet, he wants to keep his current number, but due to hassle I’m reluctant to activate his sim. 
any tips on changing number properly, I feel I did it correct as followed link when activated sim, but it didn’t change correct number so how do I do it???