B.M.
I depends on where you live. In Chicago I pay a college student $10/hour to watch ONE 10 YEAR OLD. So, $10/hour for more than one kid is a steal.
I would ask her.
You can also go on sittercity.com and use their calculator, it will have you put in your zip code and give you rates for your area.
Also - look at 'trading out'. So, if there is another mom on the team who would watch your 1 year old if you took the boys to practice that might work out for both of you! Or - look at someone at your son's school, another mom or teacher who might want some extra income.
Good Luck!