Click on this link and read about 'Get an Apple Music Family subscription" --> https://support.apple.com/HT205595
"If someone in the family group already has an individual or student subscription to Apple Music, renewal of that subscription is automatically canceled when they join the family group."
However, your children are already in Family Sharing. The above might apply to them too but just in case it does not (I read Apple's documentation word by word), you should check their subscription status to their individual plans after you upgrade. If necessary they may need to cancel their plans once they are covered by yours. Please report back so I know how to advise people who occasionally ask similar questions. :-)