Looks like you can't have a passenger anymore either. I like to have my kid on the backseat of my e-bike.
I'm not in the market for this, but I am in the market for making my cargo bike electric. Having to slow down at a traffic light and then start from zero absolutely sucks. It goes very slow, it is rough, and you won't have time. I'd use it just for that specific use-case.
If it’s for a chill cargo bike, just get a hub motor kit. It’s the proper solution, there are tons of them and they are cheap and durable.
Front mount or trailer for kid could work, but this seems inferior to your ebike.
Conversion kits that add a front hub motor are relatively affordable and easy to install. Should work with most cargo bikes if you can find one that's compatible with your wheel size. Something like: https://swytchbike.com/