IMO most of the posters are being unnecessarily pessimistic about your situation. I overstayed by 2 years by the time I applied. I did almost exactly what you're planning on doing. I got my application ready to go and went on vacation to visit my parents in Asia. Husband mailed it while I was out of Canada (and therefore in status at the time of mailing and receipt by CIC). On the way back though, I entered through the land border (I'm a US citizen). My understanding was that it would be much better to have my husband accompany me when talking to CBSA and that the land border folks were more lenient generally than at the airport. I was also nervous about being detained in secondary at the airport with two cranky toddlers. The officer at the booth was very nice when referring me to secondary -- told me to go inside and get a visitor record. The officer inside wasn't talkative at all. Just went in back to check the computer after I told him I had applied for PR. Came back and told me sponsorship was approved and proceeded to give me a visitor record for 1 year before I even asked. Like everybody mentioned, there's no guarantee that you've be allowed to re-enter Canada, but you're applying for permanent residency. The scary news article about a short term visitor who was refused for an earlier overstay is hardly on point.
Some practical points:
You've already procrastinated long enough. For peace of mind get your application in
BEFORE you come back from the UK. If you're not married yet, consider getting married. Then you won't have to collect evidence of co-habitation, which you'd have to do if applying common law. (Also, since you were trying to fly under the radar, chances are you actively tried NOT to leave a paper trail, making it harder to prove co-habitation.) The question of fraudulent marriage for immigration purposes is moot -- you already have a child together.
There's no place in the application where you'd have to state that you overstayed. Just enter the dates you entered and left, no commentary. Obviously, if CIC does the math and checks in their computer for any extensions (and finds none), they'll know you overstayed. Just no need to do the work for them or draw attention to it.
In the personal statement part emphasize that you've been living together from [date] and have a child together. There's no need to overdo it on the photographic evidence. I literally sent in 6 pictures -- 3 from our wedding, 3 with our kids (one in the hospital at birth and on each subsequent birthday). I procrastinated in large part because I was overwhelmed by the application and the thought of gathering the evidence. Since we weren't planning to return to Canada when we got married, I didn't keep records of the minutiae, like when my friends met him or phone/email/Skype records or take a gazillion pictures while we were dating. By the time I applied, that kind of beginning of relationship evidence became completely unimportant compared to the years of living together and having kids together for purposes of proving a genuine marriage. (So the procrastination wasn't all bad in the end

.)
Don't worry about your kid. If he was born in 2010, what school are you worried about? Preschool? Missing a few months of preschool (or frankly even all of preschool) isn't going to keep him from getting into the college of his dreams. Just worry about getting your immigration situation sorted.
Good luck.