You are not a pervert. You are a beautiful soul that found out what she likes, found another consenting adult that likes the same, and you are past the hard part of having that relationship already.
Also you found out you really liked it. I think you are partially dealing with cognitive dissonance (that image you have in your head of what you should be as per society/upbringing/mental image of that) and that you liked the onesie, which you didn't expect.
Also, you are putting a lot of pressure on yourself, in the sense that you are stressed due to considering yourself a pervert and maybe getting rejected by your little.
Just be kind to yourself, and make peace with that the onesie turned you on. It might be that your little won't like it as they too are dealing with cognitive dissonance if they get embaressed. Be gentle, but be open. Share yourself with them because at the end the deepest love you'll ever reach is by opening yourself up and being accepted by your SO so its a risk worth taking. And honestly, you've already come this far.
I wouldn't put the onesie out in isolation. I would print out 5 or so different styles of clothing that you might want to get them and then one by one, point them out and ask what they think about each one.
That way there is less pressure, and you can see their reaction without them seeing that the onesie is the piece of clothing that you want them to wear most.
Best of luck! There's nothing wrong with you.