I believe you can on the weekends. I know I've seen them for rent at McLeans Island, and *think* I saw them for hire out at Bottle Lake Forest. I ride during the week, so I've only driven past there on Saturdays.
Do you mean the entrance by the dump and across from the golf course? (That's the only one I know.)
You can also enter via Spencer Park, which you should *definitely* check out if you want a nice place to take the kiddo.
There's a bike and lawn mower shop (yeah, I know

) on Columbo street toward Beckenham that has bikes for hire. I don't know what their rates are like, but I figure they might be a little cheaper for being on the fringe of town.
BTW, here's a nice little website with maps of the local tracks.
On Your Bike
Let me know if you want a riding companion--unless you're totally hardcore.

I'm pretty soft myself.