How to explain the concept of fostering to a 3 year old

[MerlinsBeard]

PIL are foster carers and they foster newborns thru to preschool. Usually short term but it can turn into a year or 2 (still classed as short term apparently). Anyway, DS1 is 3 and has started asking about it. Since he has been born there have been 6 foster children, 2 lots of 2 at once and the current one. He keeps asking about teh last one that was there, lets call him A (obviously i can;t name anyway - u never know)
We have just been saying that A is with his mummy and that he stayed with Nana for a while but i am dreading when he asks why. Any clues how to approach this?
Don;t want to scare him into thinking that when we visit taht he won;t come home or that if we drop him off there that we won;t come back

just want to be prepared!

never been in the situation but what about
some boys and girls mummy and daddys sometimes need some help so while they are getting that help then their sons and daughters stay with his nanny for a holiday.

I would keep it as simple as possible -something along the lines of these childrens Mummies /Daddies can't look after them at the moment so PIL are looking after them until they can go back to their family.

i don;t want him to think that we can't look after him though. There will be times in the future that he will be dropped off there without his younger brother or times when they will pick him up from preschool.