She's a "teen" and usually, teens are "cheaper" than an adult experience sitter, but still, I think they took advantage of her. She now has last summer as "experience" and if she did any other babysitting besides for that family, then she has accrued more experience.
This summer, I would engage in the negotiation... along with her. A 17 year old, can still get "convinced"/pressured to be paid the same amount as last year.... having 2 Parents, talking to a 17 year old at the same time about "pay" and "hours" can be intimidating. If the parents "balk" at the fact that you are helping/supervising the babysitting negotiations with your daughter, remind them that she is still in High-School (I am assuming) and she is still underage.. not being 18 yet.
Most sitters, range anywhere from $10/hour and Up.
I am assuming she will babysitting them all day, ie: 8 hours??? If so, in my city, an all day babysitter is about $650/month and up. I used to babysit in my home... just ONE child, and I charged approx $550 per month... but this was years ago, and the rates has increased since then. My friend for example, with one child, pays her Sitter $650/month. But, maybe each city differs on the going rate.
Next, YOU have to decide if she wants to be paid either "hourly" or by a "flat rate."
Now, she will actually be watching 3 kids.... realistically. And, they have to be honest about it, because even if they tell her to babysit only the 2 kids... the 10 yr. old will still be around in the home... AND IF ANYTHING SHOULD HAPPEN TO HIM WHILE SHE IS ON DUTY, SHE WILL BE LIABLE... AND JUST AS RESPONSIBLE FOR HIM TOO. Even if the Parents said to babysit only the 2 twins. BE sure you know that.
I would ask the parents to SIGN a paper/contract, stipulating their agreement... that it is ONLY the twins she will be responsible for. THEN someone else has to watch the eldest child.
No 10 year old should be left to his own supervision anyway. ANY parent would know that. That is asking for trouble.
They are just trying to get away with paying your daughter for 2 kids... and not their 3rd child, so that they don't have to pay as much. They are being cheap.
Be honest with them. Ask for honest pay that is on par with the rates in your town. Ask for a signed agreement...
Stipulate either hourly pay or a flat-rate, monthly... to be paid ON time at the time services begin, and that she would only start babysitting once she is paid for the month... otherwise they could just put off paying her and make excuses. OR, you can as an option stipulate that your daughter get paid DAILY, or weekly.
have your daughter keep a time chart, on how many hours she works too. If she is asked to babysit for hours BEYOND her agreed upon hours, then it is "extra" pay.... most charge about $5 per every 1/2 hour over-time, beyond the regular work hours/schedule.
I am APPALLED that they only paid her $35 last summer! That is totally rude and cheap. Mind you, babysitting rates are typically based on the person babysitting ONE child.... in your daughter's case she would be watching THREE kids. Therefore, her pay should reflect that.
HELP your daughter get fair pay and requirements.... and something that they BOTH sign that releases her of any"liability/injury" etc., for both her (just in case she gets hurt herself while on duty), and per their kids. You can find these boiler-plate "contracts" and forms online.
All the best, hope this helps,
Susan