This is such an impossible question to answer because the expectation levels vary so drastically with each individual person.
Some people think of "riding a snowmobile" and they might equate it roughly with the kind of riding that somebody might do on a Utility 4-wheeler like a Honda Rincon. You're just riding it down a trail or you are just riding across some fields or some meadows or something likely pretty chill with your butt likely on the seat. Let's call this Grandma riding.
Other people think of "riding a snowmobile" and they equate it more with what you might see somebody doing on Honda CRF250 motorcycle. Now we are talking about racing, jumping, flinging the bike around, and more likely your butt is off the seat way more often than it is not. Let's call this Kesterke riding, named after the guy currently pushing the boundary of this type of riding, Caleb Kesterke.
So I don't know where your expectations are and where the expectations of your kids are. But I can promise you, that unless your 7 year old is the most exceptional rider the world has ever seen, they will be doing a lot more Grandma riding on that freestyle 550 than they will be doing Kesterke riding. Even for your 10 year old, if he/she hasn't been previously trained on a smaller sled, will likely still be a ways off from doing almost anything more than Grandma riding on a sled that size for quite a while.
Here is my 6 year old boy on a full size Mountain Max 600. If I think riding a snowmobile is like riding a Honda Rincon, then I might come on here and tell you that my 6 year old did awesome on a full size sled, and yours can too.
However, here are a few videos of the same kid, and a few of his buddies, a few years later on some snoscoots. Now they are really riding that dang thing and beginning to learn the skills necessary to ride a machine more like a Honda CRF250.
But the snoscoots do run out of power pretty quickly, and for the older kids, even though they can still do an amazing job of teaching the kids more riding skills, they will want more power as they hit somewhere around 9 or 10.
That is where my buddy from above got his kid a freestyle 300 and you can see how great he did on the 300.
The freestyle 300 helps the kids keep progressing in this direction due primarily to the very narrow ski stance and the light weight (370 pounds dry stock).
The freestyle 300 is an exceptional sled that offers a lot of significant things that can't be found in almost any other sleds and certainly not any modern sleds. The freestyle 550? not so special. It's basically just another phazer just like the phazers and 500's that came before it and much like the EVO and now somewhat the Blast after it. Something in the ballpark of 50-60 HP. Something in the ballpark of a little over 400 pounds. Something in the ballpark of a 39" ski stance. They all pretty much fall in that same category and they are all very similar in those regards. Still, in many cases, those are good stats for folks that are a little smaller that want a sled that is a little smaller. Probably not the best place for your 7 year old to start though, and maybe not even the best place for a 10 year old if they really are just starting out. They will still have fun and stuff, it's just that a smaller machine would have done more to help them learn to ride the snowmobile instead of just learning to ride on the snowmobile.
