I'm confused .... Is the notice to the family with the siblings terminating care and then you are carrying on with the children you have left or is the notice that you are shutting your daycare business down and everyone with have to find new arrangements?
If its the first one them I would give two weeks notice or whatever you have in your contract for notice of termination but if its the second one then I would still give the two week notice to the siblings and give about 6-8 weeks for the rest.