I can't do the small Sumatrans at all.
If they lined themselves up in order of size, shoulders back, facing the front, I might be able to work them out eventually. But you never see them all together, they move around, climb, play with sacking, and wrestle each other and they can appear or disappear in a moment. Plus of course they spend some time with Emma and then go over to Subis, or vice versa, so you can't even tell which is their mother.
My advice is to see if there is a keeper or researcher watching them and smile sweetly before asking 'which is that one?' But you can't do that too often and I'm not always sure that they know either
I'm glad it's not just me, then! I didn't have time for intensive observation which can sometimes reveal what sex they are at least but, as you say, they don't make it easy
