Yeah, don't go for the Classic/Henkei one, the transformation is a little bit complex/fiddly. One of the various reissue G1 ones might work and shouldn't be too hard/expensive to find, or there was a 2007 stuffed toy/plush version as well if your kid won't think it's 'for babies'.

Or you could just grab one of those discounted Buzzworthy Bumblebee with Spike packs clogging up KMart shelves everywhere. It's small but it's reasonably articulated with a simple-but-still-there transformation, and they're widely available at $15-odd atm.