Recently made the move to electric from a beta 50cc. Haven't found a ton of first hand experience with the mini electrics; thought I'd post this, maybe help someone else out. I've got an 8 year old. Weighs ... 60lbs?/30kg -- soaking wet. 4'4" ish, which is 135cm according to google.
Loved his 50cc but was outgrowing it. Skill-wise. The centrifugal clutch really stunk when things got muddy or steep or generally needed to avoid spinning out the rear. That, and it was very heavy for him. All was great until it started to tip a bit. Really wanted something with a clutch, but no luck in his size. We drove out to see an 80cc Beta Junior but it was too big, not to mention exceedingly expensive (used!).
Long story short: Beta 20 Mini Electric. Brand new. Had to wait a bit for dealer to do their thing.
Anyway, half the weight of the gasser. Size fits him perfect. Still no clutch but at its highest power setting the front end picks up pretty fast/easy. My boy absolutely loves it. Only real downside: battery life.
The manual says 2 hours at constant velocity but they must mean riding downhill.
36V sealed lead acid batteries last about 40-45 minutes... tops! Super fun the first 10-15 mins, then so-so the next 10-15, and limp home mode after that. For reference, my boy really likes fast steep climbs and (attempting) to wheelie. Both of those mean short high power bursts that likely do a real number on the batteries. So maybe 20 minutes of "real" riding. If you let it rest a bit you can get 5-10 more mins out of it.
I am currently looking into Lithium packs. I read good things about them, but we just bought this new, and a good pack with charger is pretty spendy. That, and I'm a little skeptical. I'd hate to dump a lot more money for an extra 10 minutes of mediocre ride time.
Hope this helps someone. Maybe adds some perspective. Kid loves the bike, real nice being able to throw it in the boot and not flood the carb (or my car), but ride time is a bummer.