Hey Crumpet - We've got walkie-talkies for that purpose. We use them in the neighborhood when the younger ones are out and about (10 yr olds). I only bought my TEEN daughters cell phones when their lives got so busy they were frequently out and there's a need to know where they are - issues like do they need to be picked up, when will they be home, etc. My 10 yr old doesn't go anywhere on his own (except around the neighborhood), where I am not taking him there and picking him up (like a friend's house, or hockey, etc.). So where is there a need for a phone?
I don't think I would make the girls call their mom. After all, she's the one with the tricky schedule and can certainly call them at will and the fact they don't spontaneously call her often may help her "get a clue" that they are little girls that are too busy doing kid stuff to remember to call.
I also agree with the poster who mentioned that the cell phones (or even phones in the bedroom) reduce your supervision capacity and that is SO important for their safety. You are right about the IM thing too. Since my kids have family out of state, they do have IM capabilities, but for the 10 year old, I decide who is on his buddy list and when it can be on, and he is supervised. You can install software to log chats and IMs btw. Truth is that he almost never uses it. I pay $20 a month for unlimited long distance on the landline so he'd much rather chat by phone.
I think a lot of the things people give younger kids, like cell phones, push them to be more grown up than a kid should be. It is too much responsibility, plus it sometimes creates problems with their peers who may be jealous "so and so has a cell phone." It's kind of like allowing them to dress like little teenagers, yes it's possible, but IMHO, not a good idea.
I HAVE teen girls, and pushing them to grow up to fast backfires in a big way! All 14 year olds THINK they know everything, but the girl that's been allowed to have all the big kid stuff early is often completely out of control as a teen. I've seen it. Mom spends all that energy playing "friend," enforcing few limits when they are young, and living vicariously through their young daughters ("I want her to be cool, popular, beautiful, have all the things I didn't, etc."), that when the hormones kick in, they have no respect for their parents and incur all sorts of problems.
Let them be kids. childhood is so darn short as it is.