Does this happen in other people's nurseries? If they are short-staffed in one room (say the older children's room) because a staff member is off-sick etc, then they cover it with a member of staff from another room (such as the baby room) if that room is well-staffed. This means that my sons's key person has twice not been there for him recently, as she's been working upstairs in another room. Not great in terms of stability when he is quite new to nursery. Of course you have holidays, sick days etc, but this policy seems to add to the instability.