Essbee - I don't know if Marslady's price is an advance fare, but they do get booked up. So book asap.
You can book your tickets online, then buy a railcard before you travel. (You have to buy them from a station.)
Also you can have two adults named on a railcard, and they can each use it individually. So if you know someone who might also use it occasionally, you could split the cost. But you do have to name them when you buy the railcard.
Also, when you're booking, check how much extra another child seat will be. (It may only be a few pounds with the railcard). That way you will be guaranteed plenty of space on the train ... sometimes the trains can be very crowded at Easter, and it can be hard work with a little one on your lap all the way.