I found it almost impossible to get acceptable tickets to fly with my daughter if she reaches 2 years old during a round-trip.
Their website (I'm talking about 2 European companies, one low cost and one standard) don't allow to proceed even trying to pay the forward flight as a child as well.
Buying tickets for forward and return flights separately is not an option because the prices are 5-6 times higher. Their offices say I should be able to buy tickets as a child, but they should try on their own before...
As an alternative they might sell the tickets directly but with extra fees due to the offline procedure.
Of course, whenever possible, you can change the dates to avoid such a "problem", but I cannot find a reason why websites cannot charge one flight as infant and another as child, when the birthday happens in between.
Among all billions of passengers every year I assume it shouldn't be so uncommon.
Why do they discourage such a travel?