Haigh, a Rosie! These are both compound nouns, so they’re declined just like the last element in both words, i.e. ‘páiste’ (4th declension, masc.). For example, you could say ‘teach garpháiste’ and ‘teach garpháistí’ (‘a grandchild’s house’ and ‘a grandchildren’s house’) or ‘teach an gharpháiste’ and ‘teach na ngarpháistí’ (‘the grandchild’s house’ and ‘the grandchildren’s house’), which would be more natural, I suppose. Notice the lenition and the eclipsis on ‘garpháistí’ in the genitive singular and plural, respectively. By the way, you can check the genitive case of most words in the online English-Irish Dictionary (www.focloir.ie) by clicking on the bit where it says which declension and gender a particular word belongs to. In the case of ‘garpháiste’, this information isn’t displayed, but you can find it under ‘páiste’.