Thanks for the comments Wendylon - it helps a lot.
W will have S during the week when he is at school. I will pick him up from her place Friday evening then take him to school Monday mornings. So we will see each other once a week for the handover. We have not discussed school holidays yet! It does worry me because W has been making comments like S does not notice her and she may as well be invisible - but then she hasnt exactly been around much these last few weeks.