I agree with Danita, don't hide any pics of their Mum. No matter what your relationship is with their mum, only talk about her in a positive way to the kids. They will probably ask why they aren't living with Mum, keep it simple. Mum is unwell at the moment and she needs to get better. If Mum was part of the decision for the fostering, then Mum asked if you could have a holiday/stay here for awhile. She thought you might like x,y,z

I have a book called "Toddler Adoption; the weaver's craft" by Mary Hopkins-Best which has some good ideas. Not everything will be relevant (it covers international adoption alot), but might be worth checking if your library has it.

Don't hesitate to ask the department, your social worker for the things you need too.