Step daughters/Do-good-Mom/clean
You start to clean the table to help Anna, putting the dishes in the dishwasher, etc.
You, casually: "You know,if you'd want I could help you babysit every now and then, help you out a little if you want".
Anna looks at you with a clear mix of embarrassment and hope. She obviously needs help but did not dare ask. That plus wouldn't it be taking advantage of you?
Anna: "Well I can't ask you that ..."
You, warmly: "No problem don't worry, I'm happy to help".
Anna, extremely grateful: "In that case maybe you could take them home from school Monday? I really could use to finish a bit later that day but usually I can't because I need to go take the girls at school and all"
You: "No problem, I'll take them home and will take care of them. When would you be back?"
Anna: "Thanks a lot! I won't be that long anyway, I still need to take my baby from the nursery at 6. So you'd take the girls at 4PM and then I should be at the house shortly after 6 or 6:30."
You, confident: "Ok it should be easy enough"
Anna, doubtful but smiling: "I don't think you realize what you've just promised!"