Theres a Dynamo hub thread best to ask there utfs ;)
I think the challenge you have is keeping it light an SP is probably a better shout, but still not cheap compared to the Shimano ones. Also remember that most dynamo are meant for ~700c wheel diameter.
Dynamo wheels for a 20” wheeled bike? A Moulton specifically.
a) practical?
b) best value option? SONs are bloody expensive