the only way i'd consider it "normal" is if she said "would you like me to pick up DS1 for a couple of hours for a play date". i have done this numerous times with family and friends - taken one child out for a while as they are more "suited" to what we're up to (park etc) or because i can tell mum just really needs some time out or one on one time with another child kwim?

if you're invited (as in mum) then i would expect mum + all kids. if one were elsewhere, well that's up to mum!