Brilliant, thank you all!
I've no experience of toe straps and the velcro ones looked like they'd be limited by where the fluffy side of the velcro stops and the hooky bit begins. But the clippy ones in Vorpal's photos look perfect, and threading them so they tighten on the outside of the footwell is genius - so much easier to stick his foot through a loop and tighten it than to fiddle with the clip each time. Yes little slots, not fixed, so could do this easily.
That'll stop him undoing them himself too. No point telling him not to. He's only a year and a half, so although he understands "foot" and "shoe", pretty much any and all requests for anything get the response "NO NO NO NO NOOO!", followed by very fierce demands for a banana. I don't think his feet can actually reach any dangerous parts of the bike, but they could probably reach some tender parts of ME
. Plus yes, he would enjoy taking his shoes off and chucking them into traffic. Or into my spokes. Or under my bum. Or absolutely insisting that I need to turn around and take them off him and SAY THANK YOU, right this minute. Basically the more of him tied down the better for the time being...