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.
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/
Also: I'd not recommend hub motors with cargo bikes. There is a reason all cargo bike makers switched away from hub motors. They're low-power and often not strong enough mechanically.
Swytch are great in marketing, not so much in engineering. Very proprietary system that keeps changing every year or so. We have one kit, but I'd not buy from them again.
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.