The word "zwierzęta" is plural (and 'not masculine-personal plural), so the right adjective form is "małe".

"mała" is feminine singular.

not at all. table for -ę ending neuter nouns

There is a small group of Polish neuter nouns that end in -Ę, refer to animals and take -TA in the plural: zwierzę - zwierzęta (animal - animals); piskię - piskięta (baby bird, nestling - baby birds, nestlings); niemowię - niemowięta (baby - babies)