You need it. I travel to Ireland very frequently and I always need it. You can use other photo ID but recently (last few years) they have got real strict about it, adn tend to really only take passports.
For an emergency situation - you can travel to NI without a passport (you can use other photo ID), then just get a train or bus down to the south (We had to do this once when one of the guys in my band lost his passport and we had a gig we had to get to! Took bloody ages but we got there!)
Saying that, I have always gone to Dublin airport, where they are fairly strict on security. Other airports may be different.
TBH, you might be alright with the expored passport, since its for a child? They could be lenient..