3rd Declension Much Simplified:
The defining feature of the 3rd declension (as opposed to the first 2) is that here we have consonant-stems (mostly). So the endings are added to that stem differently (basically, after a consonant you need a vowel)
With these nouns (even more than the others) you need to learn nominative and genitive (with article to tell gender).
So for παῖς (either ὁ or ἡ) we learn from the genitive, παιδός that
the stem is παιδ- to which we add
the endings, -ος (gen.) --> παιδὸς
(dat.) -ι ---> παιδὶ
(acc.) - α ---> παῖδα
the plurals are: nom. –ες --> παῖδες
gen. -ων ---> παίδων
dat. -σι ---> παισί
acc. -ας ---> παῖδας
Notice that dative and accusative plurals just add –s- to the case-markers.
But in dative plural the –s- and –i- reverse order, and –d- + -s- simplify,
as in the nominative singular παιδ + ς --> παῖς