I was searching for last minute vacation deal to Mexico for this Christmas holidays. Based on the Travel Search Engines review from the Consumer Research Magazine, I searched the Kayak, and the SideStep sites for flights to Cancun from San Francisco. Kayak consistently turned up pretty good deals. In fact, Kayak deals were cheaper by $250 per person on average. All the cheap flights were by Alaska Airlines (found on Orbitz and CheapTickets). I couldn't find the same flights on the Orbitz and CheapTickets when searching directly on those sites.
Kayak allowed me to only enter adult travelers. There is no option to add infant to the flight search. So, I tried the exact same option on Orbitz directly and flights from Alaska Airlines were now available. Apparently, Alaska Airlines does not handle bookings for infants, even on their website also. Finally, I called up CheapTickets sales and got added my daughter to the flights manually as there is no way to edit the flights found through Kayak.
It's pretty strange that neither the Travel Search Engines nor the Alaska Airlines caught this issue. So, if you want to take advantage of Alaska Airlines flights through Orbitz or CheapTickets beware of this issue.