Some references of the female figures in the Visconti Tarots might resemble Maria Bianca Visconti, who had been married to Francesco Sforza for ten years by 1451. Some references suggest the cards came after the fourth Duke of Milan was crowned in celebration of their tenth anniversary or in celebration of his popular appeal for the Milan citizens. If you have access to the Stuart Kaplan Encyclopedia of the Tarot Volume II, there are good portraits. But no suggestions about the childish look of the figures in the courts, as far as I can tell. For lack of a good model, perhaps they just went along with certain card/game conventions of the time?
For the use of naive figures, only suggestions follow
It's puzzling sometimes why naive styles seem to be around in these minature paintings. My only clue was when we came to the Siennese schools of painting with Giotto, the muralist, and Giovanni di Paulo, who painted Paradiso scenes in the 1400s. If you scroll down in the link below, a sample of Giovanni di Paulo's naive figures can be seen:
http://www.italian-art.org/divinecomedy/art/anon/side.html
What it showed me was a naive style was favored by the patrons when they did certain narrative scenes, so the schools of artists/artisans under Giotto or Giovanni di Paulo would have painted that way. In Giotto's case, he was actually painting to fit a particular setting.
Perhaps you will find other sources that suggest the Visconti cards weren't done in the 1400s--but Stuart Kaplan's Encyclopedia of the Tarot, Volume II and Lo Scarabeo's Visconti Gold book and deck set seem to agree on this.
I cannot find any other online examples of anything close to the Boniface Bembo style of the 1400s. I did see at a museum something attributed to the school of artists in Milan---it was a private alterpiece with the same style of yellow-haired, childish aliens and allegorical figures. I'm thinking maybe the naive style of minatures had a lot of cupid/putti/angelic figures with childish bodies and big heads might have been considered---well, cute to them. I've heard that the golden-haired motif was popular and desired among Northern Italians.
My last suggestion--the appeal of the funny big-headed cartoon characters with the tiny figures appeared in early television and movies for kids---maybe there's something to this style of art that is appealing to family audiences, so it is a reoccuring motif.
Another suggestion is the Art of the Tarot by Christine Oleson--it's a five dollar giftbook that shows lots of historic tarot cards. It might be that the styles of naive figures to be quite popular in gaming cards...