I can't speak from personally experience but helping with the baby gets old pretty quick and no matter how much you try, with a newborn, the attention is pretty much always focussed on the baby.

I would think having a 4 and 6 year old at home would help keep your daughter busy and give her some attention that does not evolve around the baby. Worst case you try it and have to cancel the arrangement after a couple weeks/months because it is just not working.