Hi Michael,
I would say that for most kids and their "first bikes", an important factor in mastering how to ride is good bike design. Weight is maybe the biggest factor (for a 30 lb kid, the Woom which is pretty much the lightest bike on the market at 11 lb, is 30% of their weight!) but also the bikes' geometry and Q factor. I have seen other kids on big box store bikes that weigh almost the same as they do, trying to learn and struggling, and it doesn't look like much fun.
Using handlebars designed for adult-sized bikes may throw the little bike's handling off significantly (and the promenades are almost a pound all by themselves). I would also put forward that the promenade is a swept back bar, and most kid's bikes are designed around flat bars. He'd likely be cramped and have a hard time turning the bars (unless you put a long stem on the bike - more weight - but even then, I think the bars would take up a lot of space). I mention all of this, because I've thought about the same thing as you for my kid. I want him on a bike that has the same aesthetic that I love in bikes... but yeah, between a bike that weighs 11 lbs rather than a bike that looks pretty cool but weighs closer to 18 lbs (Veloretti kids bikes, for example) - I'll take the trade off that my kid enjoys riding and takes pride in mastering something. He's 3 years and a few months and is already riding very independently (we did 10 km to a local watering hole last week) - that's what's been the most important thing for me. I'd also say that comfort (swept back vs. flat bars) is less of an issue for a young rider doing a few km at most.
I'd say when you get to 5-8 year old bike sizes, you might be able to Riv it out a bit more.
best of luck either way and cheers,
John