Many worry about the child falling overboard. I have to say, I can't imagine that would be much of an issue as long as the caregivers are diligent with the small child. My biggest concern with my almost 2 year old was making sure the chairs weren't right next to the balcony rail because he is such a climber. Truth be told, I still don't think he could have climbed up the rail. It is plexiglass and there really isn't anything to get a foot hold on.
The benefits for us were worth every penny spent to get the upgraded room. We weren't stuck in an inside rooms in the afternoon when he would nap and if there was anything to see we didn't have to haul him with all his gear to an upper deck to look.
I definitely think an internet search to familiarize yourself with what type of balconies your ship has before leaving would be helpful (or a good travel agent will do it for you) to make planning for your trip that much easier.
Happy sailing!