Is she doing 20 full on hours with the kids or is she contracted for that much time but they aren't always there?
Load and unload dishwasher daily totally normal
Wipe down kitchen surfaces after use definitely fine
Put away any clothes in the children's clean laundry basket also fine
Tidy and hoover children's bedroom including changing bedding once a week - this I feel is sort of borderline if her 20 hours are full on. The kids really should be helping with the tidying anyway.
The following are grey areas:
Hoover the living room once a week
Sweep and mop the kitchen once a week
If she is letting the children leave a complete tip in the house and then you come home to a mess to clean up, she should be doing something about this.
For example, if she is letting the children eat in the front room put a stop to that and remind her, hard, that any mess will need to be hoovered up. I don't think she should be doing general hoovering the way a cleaner would though. It's not abusive but I probably wouldn't ask (you'd have to nag anyway.)
Same with the kitchen floors - we come home to all sorts on the floor after the cooking has been done and it's totally ignored. We've implemented sweeping up mess and next year I may ask for a floor wash.
Ours will wash down the oven surfaces after she's cleaned. Our rule is basically - everyone who makes a mess (DEFINITELY including the children) helps to clean it up.
However, my children are only with the au pair some days for one or two hours and she is paid the same rate as if they were there all the time. If I had younger children or the time she was with them required much more focus/direct engagement I might rethink.