One is without railcard discount.
The one with railcard discount is more expensive though?
OK, so I can see how it was working it out, but I have studied the T & Cs of the tickets and the railcard and I can't see why one child has to be paying the F&F discounted rail fare. The T & Cs of the railcard state that I have to be travelling with a child, which I am doing. Where does it say that a child (or between 1 and 4 children) travelling on a £2 ticket offer would invalidate the adult railcard discount?
To get discounted adult prices you have to pay for at least one child. You can pay for an under 5 child if you want, but just travelling with them isn't enough. That scenario is specifically included in the T&Cs, so by extrapolation I'd say your eprerience is also covered.
3. Group size: At least one named cardholder and at least one child must be travelling to enable discounted tickets to be used. The maximum group size is up to four adults (aged 16 years and over) including the named cardholder(s), and four children (aged 5-15 years).
That's reasonably impressive of National Rail Enquiries to have worked out that paying the railcard fare for only one child was cheaper. I don't think any of the booking engines that actually sell tickets do that sort of calculation automatically; do they?