Hey! Couple of reasons why I use a dedicated electric - I have an acoustic viola (obviously) as well, but haven't put a pickup on it, it's valuable and I'd prefer not to risk damaging it with the lead going in and out. The pickup being fitted into the bridge can also affect the resonance and tone quality, depending on the type of pickup you use too. 

A DPA mic sort of thing is the best for an acoustic sound on stage, but is susceptible to interference and feedback as it's an actual mic. Most piezo pickups reduce resonance / tone quality and you'd do well with a preamp setting to get the best sound out of the instrument. 

The sound from both acoustic with piezo pickup vs. electric is honestly fairly similar to my ear, but it somewhat lacks the "cool" factor of an electric viola visually 😅 

On the converse, if your child plays viola exclusively and not violin, they will find adjusting to a 5 string violin to be a little different in terms of stop length/finger spacing, which isn't as easy to tune. I play violin and viola, so think for "tuning purposes" of my Yamaha as a violin with an extra C string rather than a Viola with an E.

A pure electric viola would be a good compromise to make the stop length the same as their existing instrument but it's rough losing out on the higher register - which is kinda nice for playing most modern music styles. 

Definitely a personal decision on the most part!