People can only belong to one Family Sharing group at any one time. Depending upon their age the children would have to leave, or be transferred, to your Family Group so they could share with your plan. Either that or you would have to leave your group (you can take your plan with you) and get a request to join their dad's group so you could share with everybody.
Read this support article about managing membership of a family sharing group --> How to leave or remove a member from a Family Sharing group - Apple Support
To learn how to join a Family Sharing group, read the section "Join a family group" in the document "Set up Family Sharing". This link will take you straight there --> How to set up Family Sharing on your iPhone, iPad, or Mac - Apple Support
Also of relevance: What types of content can I share with my family? - Apple Support
Do not contemplate having people leave and join different groups frequently. That won't work. You can only do it a limited number of times per year.