Up to 24th Dec, I think, there is the Live Crib outside the Lord Mayor's house, the Mansion House, on Dawson street (parallel to Grafton street) - live animals (well looked after and taken away nightly to sleep) and raising money for charity but it's a bucket to put money in. Dd used to visit very often when in Creche around the corner! (And I'd put money in the bucket once or twice but not 15 times in 1 year).
Also used there are usually lots of choirs around the Mansion House and Grafton Street area especially the last few days before Christmas. All raising money for charity normally but again money in a bucket so can be free or a few coins.
At the top of Grafton street is St Stephen's Green. Sometimes there are choirs there but not often. But there is a decent playground and a large pond with lots of duck, swans and seagulls to see. And other good park locally is Merrion Square which has a playground but no lake.
DART commuter train, Luas tram and bus fares are €2 at the moment, and 1 ticket can be used on multiple journeys for 90 minutes (on 1 mode - I don't think it's mixed across them all). And if you are likely to do a lot of travel, you can get a Leap card (similar to London's Oyster) and top it up with money as you go. But the green line Luas brings you to Dundrum where there's a massive shopping centre, dart brings you to howth/Malahide/Dun Laoghaire/bray/grey stones where there are various seaside walks/beaches etc. There's an aquarium in Bray but I don't know it's opening times or if it closes in winter. Great seafront walks in Howth pier, Dun Laoghaire piers (east and west) and Bray.
Imaginosity in Sandyford is a kids museum which is not the cheapest but great to while away a couple of hours especially in bad weather, book in advance and you get timed 2 hour slots if I recall correctly. And I know we used to go to creative and arty events (sometimes performances and sometimes hands on creative things) in the Ark kids cultural centre in Temple Bar before covid but I don't know if it's still open now. The National Art Gallery near Merrion Square has free art activities for kids but some need to be booked in advance.
The National Museum on Kildare Street is interesting with lots of stuff about Egypt and vikings especially. And the Natural History Museum on Merrion Square (affectionately known as the "Dead Zoo" is also fascinating to kids. There's also the National Museum in Collins Barracks (on red line Luas near Heuston station) which does the civil war and other parts of history, and has different types of clothing from history that kids can dress up in. All are free but check opening hour especially in holiday season.
There's a great atmosphere int he city centre on morning of 24th, but it starts to die down in early afternoon as everyone disappears home to family or gets on the road to family down the country. Shops and restaurants tend to close early and public transport finishes early as well. And there's a handful of petrol stations open scattered around the city on 25th but almost nothing else - most places serving Christmas dinners are hotels and they charge a lot for it.
And then 26th is bustling around shops again for the sales and horse racing in Leopardstown, or some matches (mainly rugby).