The tour in Glasnevin cemetery is excellent and only 1 hour long so relatively child friendly. Might be best to book. You can get into Botanic gardens through a side gate from the cemetery. For food independent pizza as suggested above is excellent and child friendly but you will need to book. Alternatively if the weather is good head for the Bernard Shaw in Fizzboro. There are loads of food trucks at the back and you can buy there and head across the road to the canal to eat. There is also a tremendous greek spot beside it called yeeros where you can get take away and head to the canal to eat.
If thats all sounds like too much trouble head for Fairview park. Good playground and Hillbillies chicken across the road for da picnic. 10 minutes walk to croke park. Fairview park playground is like a social experiment. You have leafy clontarf a few hundred yards to the north and North Strand/East Wall a few hundred to the south. Your kids can wind up playing with an oncologists son or a junkie’s daughter or both at the same time.