Although in many places around the world, it is unusual for a mother to share the same last name as her child(ren), I find it is still a novelty for some immigration officials when traveling between Canada and the U.S. (Indeed we've also encountered reaction to the fact I don't share my husband's last name).
If you and your child(ren) do not share the same last names on your passports, it is always a good idea to carry a copy of your child's long-form birth certificate which lists both parents’ names.
For children born in Ontario, if you don't already have a copy of your child's long-form birth certificate, you can order one online at Service Ontario. If this is your first request for a long-form version, the cost is $25. If you are requesting a replacement copy, it is $35. Admittedly my home filing system leaves a lot to be desired, so I've had to order replacements a few times, and each time I received it within just a few business days.
In our travels, I’ve been asked for B's birth certificate by U.S. immigration a couple of times, and I’ve been asked a few times by Canadian immigration coming home. I'm not sure what the result would be if asked and we didn't have it (they will probably just ask a lot more questions) but $25/35 seems a small one-time price to pay to smooth the immigration process.
Here are some tips for Traveling with Children as a Solo Parent