I think it also depends on whether or not the children have any contact with their birth family , or can remember calling someone else mummy and daddy
How long have they been with their foster family ? If it's been a long time they might be referring to them as M and D, even if they call them by their first names. This often happens if there are other children at home . Or the kids are at school /nursery and don't want to feel the odd ones out
Please remember that mummy and daddy are relationships, not names, and it will take a while for the children to accept you and trust you . They will transition to M and D eventually , when they feel ok with it .
Likewise I wouldnt talk about the future at all with them, just take things day to day. It's fine to say - we will come back to see you tomorrow and take you to the park
Don't try and discuss being their parents forever - it's too big and frightening for them to deal with. Unless of course the 7yo asks you .
FWIW my bio 8yo rarely talks about things beyond today, unless it's a special event he's looking forward to , like a holiday . So he might ask at bedtime tonight " is it Callum's party tomorrow ? "
But he won't have mentioned it all week, he just thinks about today .
HTH