I can only give you my experience with this subject; so here goes.
When I opened up my dayhome, I had 3 siblings for the summer. They were all 4 and up, and matched my own kids in age so playing was fairly easy.
Beginning in September, they moved on (I couldn't accomodate mom's hours), and I now found myself with 2 before and afters, 1 kindergarten, 1 f/t toddler and a number of p/t toddlers. I HATED it.
The experience made me realize that I will only work with one age group or the other. I found it very difficult to find activities that everyone could do together, or alongside each other. If it was good for the small ones, the older ones whined or got bored. If it was challenging enough for the older ones, the little ones couldn't do it or had not a speck of interest.
The older ones don't nap, so you don't get that "downtime". It's tough to make them rest, most of them, because they haven't had to at home for years. The ones I had didn't have a grasp on how to stay quiet during the little ones naptime, so I was forever shushing them (and felt like Miss Meanie No-fun) or they were waking up the small kids.
Feeding them wasn't too bad, but they do seem to be pickier as they are a little older. I always got the sad/neglected faces when I had to say "No, this is what we are having today. I am sorry it's not your favorite, but I am not making something different." Keep in mind that you will be going through more food. You may be surprised, especially if your kids haven't reached that stage. I am still surprised at how much my growing pre-teen boys can pack away!! LoL
I also found that after wiping bums and noses all day, hauling toddlers around; I had very little patience for **whiiiinnnnne** "He said thiiiisss" or "She's not plaaayyyinnng riiighhht". I would rather deal with the sharing issues of two year olds, frankly.
On the plus side, when they are playing, you pretty much just have to supervise and only occasionally intervene. Depending on the kid, they may be totally happy with a book, and some outside time. No diaper changes is pretty awesome, just be ready to wipe up the puddle of water around the sink that seems to appear when school agers wash their hands. If you need a couple hours peace, they are usually appeased by a movie and will actually sit and watch (unlike most of the little ones). I LOOOOVED going outside with this group, as they found things to do and I didn't have to chase after them like the small ones.
Essentially, both age groups have their perks. I personally will only do one group at a time. In the fall, I will not have any little ones left and I am starting an out of school program. I am excited, because I have a bunch of activities in mind that I never could do with the littlies.