Absolutely book ahead; you would not want to go all that way and find it was booked. Probably you can book on the day -- but why take the risk. Note on the web site which days the carousel is functioning for rides -- you don't want to miss that and except in high summer it only runs limited times. We went with our granddaughter in April and at that time was only open on Wednesday and Saturday. the snapshots are from an earlier trip my husband and I took -- when my granddaughter saw them she insisted we had to go and so she and I did a day trip from Paris this year.

https://janettravels.wordpress.com/2019/09/13/les-machines-de-lile-nantes/

This is a really cool place -- and actually watching the elephant from the ground is better than riding it -- but you will want to ride it as well.