I think OP is asking family's to comit for a year. I don't know if many/any would do this. They may have plans to have another child, may loose their job, may not like you/your program, their child may not adjust well. As a parent I would not comit for more than a month on paper (which is the notice I require from families) but I would usually know in my head e.g. they will stay until school or until Aug when baby #2 comes etc