Don't worry about the car seat. Law doesn't require you to carry a car seat while using public transport (both in the US and Canada), be it bus or a taxi. I have lived in both countries (for more than 7 years combined) and have travelled with my son many times in public transport without carrying a car seat. Never had any issues. Not that I was taking any risk. I did my research online before I did it. You can carry your kid on your lap or make him/her sit alongside you. You can surely use the seat belt though if your kid is tall enough but if he is still short then don't worry about it. This is totally a non issue, in my personal opinion. Just relax and come with a free mind.
Here is the snippet...
"What are the seat belt and child car seat requirements when I am a passenger in a taxi?"
Taxi drivers are exempt from the child car seat and booster seat requirements when transporting a passenger for hire."
Regards,