Create checkbox tree with lazy loaded children

I m trying to make a tree of checkboxes, i m working with angular and firestore db, the parents and child nodes are related with an id in the children docs, so when implementing this tree i have to load children async on demand from db, the main need is to get all selected children from several parents;
Scenario
1- check a parent so all children (async) get selected and get checked if we expand the parent
2- expand a parent (load children) to check some children
3- get all checked children to perform a task

I'm stuck with this task for a couple of weeks
I need a proposition or a solution that works in this case, PLEASE help.