Do I need a passport for my 10 month old to go to the Canadian side of Niagara Falls? Will his birth certificate be enough to get back into the US?
Presuming that your 10 month old is either a US or Canadian citizen (on the grounds that they likely wouldn't be in the US without a passport otherwise!), the answer is yes - only a birth certificate is required.
The Western Hemisphere Travel Initiative which covers land travel between the US and Canada allows for children under the age of 16 (and in some cases, under 19) to cross the border in either direction using only a birth certificate.
Note that if you are travelling without the other parent listed on the birth certificate it is always a good idea to have a letter from that person stating their consent for you taking the child out of the country. Technically this isn't needed by either the US or Canada in this situation, but it can make things much easier in the event that immigration has any concerns that you might be attempting to abscond with the child.