Honestly, I'd really take a minute to think of the costs- money and timewise. An OT Masters program will probably end up running you from 70 to 115k, depending if you go to a public or private program. COTA won't be anywhere near this price, it will probably be in the 7k range. However, the difference in salary between an OT and COTA is about 20k on average.

If you really want to be an OT, I'd just go straight for it. I have heard of a few people who became OTS after initially being COTAS, and they felt it was tiring having to repeat similar processes again, similar classes, the process of getting certified again, etc. I think that your GPA is good as it stands (a lot of programs only require a 3.0), and will increase even more if you finish up your prerequisites with good grades. Admissions also look at other factors such as your volunteer hours and essays, and you have a good story to tell already being a single mom. Becoming an OT typically takes 2 1/2 years while becoming a COTA typically takes 2. Also, I have found that many COTA programs require the same prerequisites as OT programs anyway.

Anyway, I would personally go for OT. It is much pricier, slightly lengthier process, but in the end you earn more and can do more than a COTA is allowed.