bear in mind their Christmas is January 7 next year.
I'd be a bit wary of taking young children. It's a fairly poor country and I don't know how well they'd handle it if a child fell sick. Apologies if I'm libelling them - someone else may be able to give a more detailed opinion.
I was there about three years ago and many of the Lalibela churches were covered in scaffolding outside. I don't know if this is still the case. Inside, they have fleas.
I don't want to put you off. It's a wonderful country. Lalibela is great. But it's quite hard work, with usually a fair bit of bus travel and not very fancy hotels, and personally I'd say it was better suited to adults.
Some info here:
http://www.guardian.co.uk/travel/ethiopia
Several operators do tours there including Jules Verne
http://www.vjv.co.uk/destinations/africa/ethio pia/