At Ely you can buy an Anglia Ranger day ticket from the machine but not the three in 7 rover.
You're right. I just tried it and it would be fine for either an Adult or Child ticket, but to get the discounted Child ticket when accompanying an adult, it says to use a ticket office or the guard.
The machines at Ely sell tickets for today and tomorrow. For other dates you need to use a WWW site. The front page also has an icon for a Greater Anglia smart card, but when I clicked on it, the only option was a cancel button.
I would love there to be smart card season tickets. My kids have had season tickets to get to school in Cambridge. Lose one of those, and they will replace it for £20. Lose the second one, and you need to buy another one at £500+
Also when Cambridge North opens, I'll be using the train ~ 3 days a week, where a traditional season ticket doesn't make sense.