I am usually the bargain queen....my kids think I am unbelievably cheap....and I prefer to think of myself as thrifty...however the thought of flying standby with an infant makes me queasy....I would absolutely buy at least 1 ticket for mom and infant. If this is a family with 2 adults and the rest kids I would not do it either....
I would also rethink the Thursday night thing....flights in almost all markets to MCO fill up in this order...Fridays, Sundays, then Sat and Thursday evenings...if you need to fly standby I would look at a mid week fkight (Tues and Wed mid-days are the last to fill)
SW is running some great fares for late August now and a lot of the other airlines are matching those fares....I would come a lot closer to looking for a bargain fare even if we had to drive a little ways to get it, than I would trying to fly stand by with an infant.
That said....if you do not have any other choice and the other 4 passengers were travel saavy adults....I would buy Mom and baby a confirmed seat and look for slowest flight times for everyone else (and have a contingency plan for who flies when there are only 1 or 2 stand by seats,,,,cause it would be pretty rare for all of you to get on the same flight)
Remember...if you are going in a day or 2 early just to use the vouchers the $$ you spend in those days for lodging and food might help pay for confirmed seats on the planned day of arrival.