We've no doubt contaminated Mars already. Our landers and probes still carry terrestrial bacteria, despite out best efforts.
Those aren't our "best" efforts anymore: since discovering the mistake, we've been working on better ways to sterilize. Colonists can take precautions and follow procedures while alive to avoid it, and since they'll have to their waste and wear suits outside, the risks are further minimized. I realize now they will probably have some method for disposing of or storing bodies for when people pass away, too.
But forward-contamination from adults aside, I SERIOUSLY doubt they'll be breeding. Look at their planned colony: a bunch of tiny rooms connected by crawlspaces. Until you have the technology to build a LARGE facility, not as in total volume but containing individual spaces of large volume, it would be inhumane to raise a child there. Children need room to play and exercise, and just putting them on a weight machine or treadmil isn't going to do it. You oould saddle them with videogames and movies, but that's not healthy either, if its all they do. The sheer wonder of being on mars would just be boring to them, as it would be all they know, not having the cultural experience of Earth, so that won't do either.
Child-sized space-suits wouldn't do it, as first you'd need to develop them on earth (since testing the prototype on mars with a real child is also inhumane), the child would need extensive training in its use before ever going outside (which might take years, and still end with a fatal mistake), and they wouldn't be able to stay outside long enough to get good use, nor would it likely allow enough freedom of movement for proper exercise.
Speaking of training, that would take time, and before that set in, the child could cause serious problems. Try baby-proofing a space capsule: wayward fingers, drool, vomit, or waste could do a lot of damage. And you can't just restrain them all time: it would be cruel and they need to learn how to walk.
Basically, we likely have the technology right now to make a long-term adult habitat on Mars, but we'll need serious advances before we can build a colony that supports children.
It would be better to try raising a child in prototype habitats on Earth first, in places like Antarctica or wherever. At least if the test goes awry the child can be moved somewhere safer.