Hey Erin, I use an ovulite microscope. I think it cost me $30. The magnification on it isn't as strong as some but you can definately see ferning when it is there.
I use my microscope as the 4th indicator to determine O for me. I find it a little confusing, like I got some ferning (I called it partial but if it would have been around "O" I probably would have said it was full ferning.
: ) at cd4 while still having my period and then today cd5 none!? I think cm, cp and temps are probably a lot more reliable until you really get the microscope system down. I have looked into a lot of microscopes for my education and it still confuses me. It is real cool to see the ferning pattern when it happens and it definately made me realize I had a very late O when I wasn't charting temps cycle before last.
All and all I think it is good to have but not really necessary if you are checking cf, cp and temps. Unless, those don't point to a definite O time, then the microscope can help with those cycles.
The children's microscope would probably work. You want at least 40X magnification. The only thing you may want to think about is that it isn't really efficiently portable and if you do any traveling the small microscopes are great to thrw in your purse/bag!
I hope I didn't confuse you. I think I confused myself.:LOL You have to forgive me I have been up since 4 this morning. My dog finally had her puppies! 8 so far, and I think she is done. Phew!