Option 1 - In the bin, forks over the front edge, with the front wheel removed and stowed inside bin. Front two kids won't fit, and you'll have to mind that the steering isn't limited on the cargo bike. Advantage, second bike goes on the other side.
Option 1b- same as above but hang it on the outside of the frame. A short support that crosses the top of the bin would provide somewhere to hook the frame onto. Pictured facing backward, but might go better the other way. The inboard pedal could be a problem.
Option 2 - Extend the bin's frame and lean bikes on that. Downside, its wide and involved modding your cargo bike. Some kind of insert into the end of tubes might be adequate. Upside, still carry 3 kids.
Option 3 - Add a rear rack (yellow) and strap the bike there
Option 4 - No picture. Put the bike in a backpack like a tramping/hiking frame pack and carry it that way.
Option 0 - Three years is probably a bit small to be riding on the road, but riding a bit on the footpath may be ideal - depends on your location. I like this one!
Upside, more room in the bin. Kid gets exercise before you get there. Sets a good example
Downside, you'll have to think for him while riding, so it will be extra slow. Could double/triple your travel time.
All these answers will require some level of padding in appropriate places.