I tried (admittedly not very much) to see if I could find a place online that let me actually read this book. I came up empty handed.
In all honesty, I doubt I'd find it offensive.
I agree that a lot of kids find butts funny. I have four kids, they all laughed about butts. Years ago, my one middle daughter (she was seven or eight at the time) had a favorite knock knock joke about butts that cracked up her siblings every time. The joke went, "knock knock", "who's there?", "butt.". This was the humor in my house back then.
I also agree that the kids probably enjoyed the book. I'm all for kids reading and yeah, if it makes them laugh and gets them interested, I say no harm, no foul.