Some heavily discounted fares do not have the applicable child discount (i.e. the 'getaway' fares offered at the weekends).

As to not telling the reservation agent that one of the passengers is a child, unless you get an upgrade when you check-in online then when you check-in at the terminal, they'll still know.

My 5 yr old has also been upgraded from Econ to Biz on a number of occasions whilst travelling with me on short and long hauls so it does happen.