Demamma,
A Double Mane, or DM, is a mutated gene that gives a rabbit wool fur on certain part of their bodies. Your daughter's sweet rabbit has it on the top of her head. Just like the Flemishes posted, have it on their faces. The Breeders that I have joined with have created, with the help of the mixture of Netherland, a Mane which encircles the neck and also the skirt (lower sides to rump), which some thought made the Lion head look sloppy. That has been the crux of the whole Lion head acceptance into the ARBA. Who liked the Single Mane gene, and who liked the Double Mane gene on the Netherland Dwarf.
The DM gene is very strong. Rabbits with only a Single Mane gene may have long whispy hair, like your daughter's lop, or will sometimes loose that wool when they reach an adult. Or hold it with a little hair on the side of the face and on top of the head. Cute, but hardly a real mane. Just my opinion.
With getting any new breed accepted you have to show the full generation carries the same type. Grandfather, Father, Son, for example. You probably already know this from talking to your Rabbit Judge friend. The singles were nice looking at Jr's., but lost it at adults. The DM's, labeled "dust bunnies", or "uglies" at Jr. status, held that mane and skirt at Sr. My Neville has not lost anything. My Ugly has turned into a Beautiful Lion.
There has been a great turn for the Lion heads and their acceptance status. The ARBA officials and those working hard to get this rabbit established have agreed with the Skirt length at Jr. So that the Jr, who turn into those awesome Sr. have a shot. That's because of the last COD try.
I don't see how this DM gene would ever hurt the ND, as the ND has already been established as a True Breed. The mixture, as all who have posted is a good thing to try, has led to the Lion Head that I see today. And many others as well. Exceptionally beautiful unique rabbits in my opinion.
If you want to experiment and play with the DM gene, my suggestion is that you get a Quality Buck or Doe that has that DM gene for generations. Though the DM gene is passed and every kit will have a single gene (bred to another without it), you would need to then rebreed those kits back to the original DM holder. This brings on the more wool which the rabbit will hold onto into adult hood. My Luna and Neville are from 3 generations of DM gene, so I know the kits, and I've seen the nekked spots of a DM, will have that strong Wool gene.
If you want to sell your DM rabbits, mixed with whatever you choose, as pets, you would have to make sure that the Mane will stay into adulthood. Pet owners, I would think, would be more upset that their rabbit stopped looking like the one they bought as a 8 or 12 week bun.
Wishing you luck. Hope some of this helps you understand the DM gene. A unique mutation.
Karen