You need a piece of fabric about a metre or more wide, by roughly the length you want the top to be, a light stretch fabric being ideal. You also need some stretch bias binding or T shirt rib for the neck and armhole bindings. Or you could make your own binding from the main fabric. My fabric was a silky finish, but any light knit that will drape will would be fine. A yard wide might be a bit tight, depending on the child, but most fabrics come in about 45" or 114 cm wide plus. (This is for a small child up to 5 years old, but you can increase the dimensions as long as you have a suitable pattern. See ideas below.) It also needs to be about 18" deep or 45 cm plus. A large adult T shirt might also have enough material. As you can see from the skirt I made mine from (which is folded in half in the picture below), this was on a slight curve. But a straight piece of fabric would wortk fine - you just need to cut the hem into a curve.
Ideally, a fabric that doesn't need hemming, like a knit, would work best. My material was already finished, with a slightly frilly edge. A serged edge or even a zig zag edge would also work, But do you want something that will drape loosely, especially at the back. If you were going to use a non-stretchy fabric, I would aim to cut it a bit more on the bias,
So, having found a suitable piece of material, you need a bodice pattern, with a back and a front. I used Climbing the Willow's basic bodice top, a free PDF pattern, to make this top. It comes in sizes 18 months to 5 years old. I used the age 5 for this. But there are several other free PDF patterns that you could use if you wanted a different size. For example, this peplum top from On the Cutting Floor comes in sizes 1-8, and you could just use the top part of the bodice and extend it. There is also this one from Imagine Gnats, which is available up to size 14, though I haven't personally used it as yet. For a two year old, there is this lovely swing tank top from True Bias, which would be perfect. (I've also stretched this pattern for a three-year old, and you can see the result on this post (near the bottom).)
Or you can draft your own pattern, if you feel adventurous and don't want to be constrained by sizes. There's an A-line dress draft from Frills and Flares, which is fairly simple, or this one from Stitch and Pink.
The plan is to make the top without any seams, just wrapping round at the back. So, as you can see from the layout above, I put the centre front on the fold. I then slightly overlapped the top of the side of the back (as I wasn't going to have a seam, I reduced the pattern by the seam allowance) but I FLARED IT OUTWARDS at the bottom of the side seam. (If you were to use an A-line pattern you probably wouldn't need to do that, for example the Frills and Flares pattern, or the True Bias pattern, as they are already flared.) Then, I duplicated the back, and pinned that on as well, also a little flared. Then I cut out around the whole pattern neck and armholes (but not the bottom). On the assumption that you are using rectangular material, you will need to cut the bottom edge about 5" longer than the centre front, curving upwards to meeting the side seam of the second back piece.
When you open it out, this is what you get. You could then finish the whole of the bottom hem by serging or zigzagging. This will probably give you the slightly frilly effect l already had. And now, you can proceed the same as on my earlier post, to put the top together.
This makes a really cute top. My grand-daughter oves it, as it's very easy to pull on and off.