This is one of those comments that some people will applaud me for and some will hate me for... but the internet is not always the safest or most private place in the world. I think a 10 year old is old enough to start learning how to stay safe on the internet, and to ignore the bad language and such. I would suggest this site is not exactly the worst behavior on the internet. It's not 100% child-safe, but relatively speaking, it's not bad. And in a lot of ways, she probably would not see much of anything here that she won't run across just out in the world in general. I don't think you can protect kids from everything.
And remember that you can see what everyone on here does - you can go into anyone's profile and see what games they've been playing and with whom. So if she's only supposed to be playing with you, it's not like she can hide it if she's playing games with other people, right? You could even get to know some people and make an approved friend list.
(Does the hotseat mode work for just a random person on your own account, as opposed to the other person having to have their own account? I know they made some changes since the time that was free and in beta.)