https://en.wikipedia.org/wiki/Queen_Victoria#Issue

There really is no other interpretation of the description. It lists all the people in the photo and everything adds up, except it places a young Prince Alfred in a strapless white dress with a nice big bow at the back. Was this a joke? I'm a little surprised that they'd do this to a Prince since he's supposed to be taken seriously by his subjects, but seeing as how he's the second son and he's also quite young at this time I suppose I could see how they might do this sort of thing as a joke. But then why would they want him painted and remembered this way? Especially while the rest of the family looks perfectly normal and respectable.

Edit: Since the comment below leading to a similar post seems to mostly answer this question, I'll add in a second part for anyone that had more to contribute. It seems to be considered "children's clothing" which had the intention of being simpler to change or use the toilet. With that said, wouldn't they have changed their little princes and princesses into male or female clothing for a portrait or painting? The world is going to see them and remember them by these paintings, I would have expected them to dress them up in more regal attire and then go back to living in more convenient clothing. It seems like nobleman's clothing was rarely designed to be convenient, it was more about showing your status. I'm sure that sentiment would carry over to a family portait, wouldn't it?