Why is this "pracovala" here ? There are multiple girls, so why not "pracovaly"?
Because it is not TA dívka, but TO děvče (neuter), therefore "ta děvčata pracovala".
You can hear "ty děvčata pracovaly" in colloquial Czech sometimes, but it is NOT Standard Czech.
I would like to now if "dítě" becomes feminine in the plural? Thx
Yes, "dítě" (neuter) changes gender, and the plural "děti" is feminine, indeed.
The only other two nouns that do this are "oko" (eye) and "ucho" (ear), also both neuter, changing into feminine in the plural: "oči", "uši".
If I used pracovaly with ty holky would that be correct?
yes, holky pracovaly, děvčata pracovala