I think it would be easy to play medical science games to try to find psychological research back and forth. However, we'd probably have to spend quite a lot of time coming to agreement on the definition of trauma.
Instead, it may be easier to find a "safe" level of scaring that your husband can agree with, and stay there.
When you scare someone, you get a response out of them. You get to see that response on the surface, and with the experience of a lifetime of being a human, you can make some half decent guesses as to what that response is doing to them deep in the core (where trauma could be a valid concern). It's hard to prove you're not doing anything to them, but you can argue that you're not doing anything unreasonable by looking at the rest of their life. When they're going about their day to day business, do you see similar responses out of them? How much of this "trauma inducing activity" occurs in their day to day life, without you? Does your nephew skin their knee and fall apart crying? That's certainly more traumatic than getting spooked. How do I know? When you spook them they giggle. When they skin their knee you can see they truly got hurt by it. If they can shake off the deeper trauma of a skinned knee within a minute or two, they can certainly shake off getting spooked!
So if they skin their knee every now and then (which my kid certainly will), you can be quite certain that life is already gearing them up to deal with trauma way more brutal than your mere spooking games. They're becoming adults, one day at a time.
On the other hand, if your nephew is a fragile little butterfly (which it doesn't sound like is the case), and lives a sheltered life where nothing ever disrupts them, then you might need to be more careful. This child may be one that needs a bit more care, and less spooking until they are ready to take on a bit more of what life throws our way. If your nephews are special needs children, then there's a decent chance that your instincts about what is traumatic and what are not may not be so accurate when it comes to them, so you may want to intentionally play it safe. The parents may be able to help there.
Fit your scaring games into the child's life, and you'll almost certainly do well by them.
One particular sign I'd look at is in their response to being scared. Obviously you hope they laugh at it. If they don't laugh, that's a bad sign you're crossing a line. However, watch how they laugh. Do they laugh while trying to get closer to you or otherwise connect with you, or do they have that nervous laugh where they're trying to distance themselves from you? If they're connecting with you after the scare, you know they're overcoming it. They saw the spook, it shook them up, and then afterward they wanted to get closer to the one responsible for the spooking. That's almost certainly healthy.
If when you spook them, they try to withdraw from you afterwards, that might suggest that your husband is right. If they are uncomfortable with the person who is spooking them, that starts to suggest that you are affecting the way they view you (and by corollary, other people) in a negative way. That would also be a good sign that you should stop.
A good sign that you're the one who is right would be if your nephews start trying to scare you in return, but can't seem to do it because they've got too big of a grin on their face while doing it!