I fly Southwest at least four times a month and no, it is not worth the money.
With a three year old, they will allow you to board after group A (which includes A 1 - 60 or 60 people). That will give you plenty of seats to find together as a family and plenty of overhead space if you need it.
I highly recommend checking in online (or on your iphone as I do with the SW application) EXACTLY 24 hours prior to your departure. If you leave 9am on Monday, check in 9am Sunday. The A-listers will be pre-checked in so you can't do anything about that but you will be the first in line after them which is typically in the A group (or the first 60 people).
As for getting ahead in line for security with that extra money - it is a crap shoot! As an a-lister myself, I have found that the regular line is sometimes shorter than the 'quicker' line and even if you use that 'extra special line', the employees do too and they are priority. So you wait for the employees, then you only have one TSA agent checking all the folks in that line. THEN you are usually relagated to just one x-ray scanner and they seem to have put the fancy wave scanners in that line so you stand super still for what seems like 5 minutes as they xray everything. I avoid that line now...