Corrected sentence:
Three fun things in one amazing book: a first farm encyclopedia, a sound player with animal noises and tunes, and a double-sided playmat.
Why this is better: I used a colon to introduce the list so it reads clearly, and I kept each item in the list in the same grammatical form (noun phrases).
Short version for a 6-year-old: This book has three fun parts — a farm fact book, a sound player that makes animal noises and plays tunes, and a playmat you can use on both sides.