Even if they still exist, they really are not a great deal anymore. Haven't been for years - maybe that is one reason they don't exist anymore (if indeed they don't). You pay per segment/flight number. You have some flexibility with the VUSA fares that you don't have with normal fares, but if you can plan your trip in advance, the advance-purchase tickets are almost always less expensive.

And note - a 2 yr old is not a "baby." A 2 yr old pays full price for domestic airline tickets and 50-75% of the adult full fare internationally, depending on airline and market.