We had a situation like this arise, where one child was able to do something, and had already done it well and had REALLY helped out, but was still technically under the age limit. We'll call him VHK: Very Helpful Kid. The powers-that-be wanted to find a way to ensure that VHK could keep doing what he was doing (because he was a huge help and they didn't want to lose that), but put some conditions on participation for others his age to make sure that they were taking the volunteer job seriously and weren't just going to goof around.
What happened is that they passed a couple of rules for kids under a certain age. First, the kid has to volunteer a minimum of X hours WITH a parent supervising before he/she can work alone. (This will likely weed out the kids who have nothing better to do for an hour and just want to do the volunteer thing b/c it sounds more 'fun', or weed out the parent who has the volunteer commitment and tries to have the kid do it instead. ) Second, there was an job duty quiz that the kid has to take at the end. The questions are situational, like "I'm allowed to do X when I'm working; true/false" and "If X happens, then I should do: A, B, C or D.
So our VHK was basically grandfathered in because he had already put in loads of hours and had already proven himself (and he helped create the quiz, which helped b/c it was from a kid's point of view.)
For your situation, you could grandfather in your older kid, but say that your younger kid needs to work WITH his parent for X number of hours. If the parent has already lied and already doesn't watch her kid, then when you put the responsibility back on her that she must work WITH the kid (like the kid is within arms reach at all times or it doesn't count as working WITH him), I'd wonder if she'd decide it was just more trouble than it was worth.
From our experience, I know all too well how you can't use age alone as a means of deciding whether or not someone can do the job. So you have to figure out what rules can include the right people and exclude the ones that aren't right for the job.