Our former nanny would have jumped at this job. She was actually actively seeking this sort of arrangement before she found her current placements, but then again she is very experienced and looking to maximise cash.
OUr arrangement was five children in a share (not full-time mind). She was assisted by an au pair but the reason for this was that the older children were not in the same school/nursery and also my DD1 had activities she needed to be taken to. I think she could have done it by herself (just) had they been at the same school and with no outside activities that they didn't all go to. You will need to be prepared to make some compromises and I would say that the arrangement will have a relatively limited shelf-life but might get you through that incredibly expensive in childcare terms pre school period where you often need a nanny to keep the show on the road.
These are the things I think you need to consider.
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be very very organised - meal plan for her, make sure all the stuff is in the fridget that she needs. If you can freeze stuff at the weekend as well, but don't expect anything to elaborate. You CANNOT go all annabelle karmel on her in this situation. Have everything she needs (nappies) out and organised well so that she isn't struggling to find stuff.
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Do't expect too much in the way of cleaning. Realistically I don't think she can do laundry.
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Good idea to keep the older ones in nursery together for a good period of time they are together. Is your preschool term time only?
Things that will get harder later on is if they are in different schools, with different after school activities (if they have playdates later, the other parent/carer will probs need to transport from school), and homework regimes. At this point I think you will need to go it alone.
Our children enjoyed the arrangement immensely. As they got older, they entertained themselves well too. But as I said, it was not a full time arrangement, so you may get fewer years out of it as it might be that much more exhausting.
But honestly, I think it's doable as long as you accept there will need to be compromises on your part. If you or the other family are manic about adhering to a special schedule/meals or have different views on parenting/activities then I think it will make your life v hard but not save that much money.
I am assuming you don't have space for a live-in?