Hi Mark - I was unwell after returning from summer hols

Sorry for the delayed response.
You can add custom fields to the user profile in Joomla 5 to hold information about children. This could be a sub-form field which would allow multiple entries i.e. each entry would hold all relevant information about the child i.e. you could add a child at a time rather than having multiple names, medical infos, contacts etc.
We don't currently have a 'select one of several custom field value' field type but could easily add one.
Let me know when you are ready to go and I can code that up.