I have a Lochlan low whistle. It was made by a Greg Russell from Newfoundland who I think might even be in this forum. Anyway I kind of touched on this in my other post, but I wanted to pull it out.
I know about the pipers grip and all that, but I’ve seen people with small hands playing Low D’s on YouTube and I really want to be able to figure this whistle out if I can? Does anyone have any tips?
It will be mainly down to practice, I have reasonable sized hand, but I couldn’t cover the stretch at first, some gentle hand stretches helped me to finally be able to play them.
You could, of course, use your little finger, there’s nothing against it, just not the regular way, but if it allows you to play it, why not. 
Also theres no one ‘right’ position for your hands. Try rotating your hands a bit and moving them in or out (towards or away from the whistle) to find the best position. But what you find as your initial favorite position may change with practice.
Unfortunately this whistle kind of just forces a certain position for me because I have to play so far down on my fingers so they actually cover the holes. So I can’t do much adjusting.
It’s difficult to offer suggestions without being able to see how you’re currently attempting to hold the whistle. If you could post a photo you’d likely additional helpful responses.
Also, many people won’t know what the hole sizes and spacing are on that particular whistle, or the size of your hands and fingers in relation to those factors. As someone who also has small hands, I can say from experience, all those details matter. For example I have run across a few low D’s that had one or more tone holes that were wider than I could seal with my narrowish fingers.
Bottom line, it might be that you can make some changes to your grip and the problem will be solved, or it could be that you’re going to need a whistle with smaller holes and/or a close hole spacing. The more info you can provide, the more help folks here may be able to provide.
You also don’t necessarily need to hold the whistle completely vertically. Sometimes, tilting a little toward your lower hand can make the reach easier.
You could consider adding a key or two! There is a whistle maker in New Zealand (name escapes me at the moment, but does sell on Ebay) who makes nice & functional plastic keys for low whistles.
They’re the sort of things you could make yourself if you’re at all handy with basic tools.
I think you mean this guy:
https://www.drjohnlaurence.com/takahe-flutes
That’s the one indeed! I have one of his keys. Comfortable, well designed & executed, good functionality with the obvious caveats.
Susato makes Low Ds with keys so anybody can play them fine no matter how small their hands.
This is the best video I’ve seen demonstrating how they play, and work, from a fine Japanese whistleplayer.
He demonstrates Susato Low Ds with no keys, one key, and two keys, the pros and cons of finger-reach and how the keys impact traditional Irish ornamentation.