Háizi zhǎngdà de guòchéng zhōng miǎnbuliǎo fàncuò.
Children inevitably make mistakes while growing up.
Tips
grammar
免不了 is always followed by the inevitable thing. It's similar to 难免 (nánmiǎn), but 免不了 is more colloquial while 难免 is more literary. Both mean 'unavoidably.'