Translation:Your daughter won't break it; it's plastic.
Such sentences won't be present unless intentionally expressed.
The moment you change 'la' to 'le', the following sentence/phrase/clause must also be changed accordingly to match the gender.
"Ta fille ne le cassera pas, il est en plastique", would be the translation for that case.
If you don't change the "elle" to "il", the sentence will mean that, "she is made of plastic" and not "it is made of plastic".
The translation is "conditional" and for the following phrase in French ....Ta fille ne le casserait pas, c'est en plastique. The correct English translation should be .... Your daughter WILL NOT break it; it's plastic....for the phrase which is a simple future tense and not conditional.