Swimsuit Question

Hey! Im wondering where I can get cute one piece swimsuits in a girls 18/20 plus or a conversion to a womens size that is appropriate. Looking for a little girls style, if at all possible with ruffles at the bottom. Thanks!

Have you done a SEARCH on Ebay ?
You don't have to bid/buy but you can get an EYE-FULL tower of what you are looking for
and see what the market pricing is.
Also consider a leotard as an alternative.
For sizing in mature (pleasingly plump) look at Lane Bryant website or other major retailer.
The sizing 18/20 is already a women's size.
Women's (misses) EVEN sizes run 00 to 26 - the 16 up factor is what you are looking for.
Young Misses (Juniors) are ODD sizing 01 to 19 - 15 up is your ball park area.

HINT Rule of 20 - your waist less 20 equals your women's sizing plus/minus a size.

I think the OP meant girls size 16/18, which would be around a small or medium in juniors or misses. I am actually looking for the same type of thing, but for clothes. I would love to have "girlier" clothes, as in little girl designs.

The issue becomes that girls sizes are meant for prepubescent shapes and juniors and misses sizes are meant for more full-figured ladies. So even if you are small enough to fit into them, they may be too tight in the chest/hip or if they fit in the chest, too loose around the waist. I think for swimsuits, it would be more of an issue with them being too short from chest to hip and also having your breasts popping out. It might also be cut too slim in the front and back for appropriate coverage if you were planning to go to a public pool or anything like that.

Definitely let me know what you find out!

+1 to what photo girl said.

I've had pretty good luck with size 14 with 36-29-35 measurements, but I've mostly just bought shirts or skirts, and I'm not particularly well endowed. I will say the one girls' dress I bought fits more like a tunic (which, admittedly, has a desirable effect when worn with a certain undergarment). It seems you would want to do this shopping in a physical store so you could try things on first.

I bought a one piece swimsuit from target. It didn't have ruffles on it unfortunately but it was rather childish.

I do believe OP meant 18/20 as that size does exist. I recommend target.

The best idea is to shop a store with a physical presence using a tape measure and noting the label size data and the final test the fitting room.
Online (Ebay) can work but you need the full co-operation of the seller with a tape measure. I shy away from sellers that tell me to look up the
manufacturers size chart. With the Asian sellers at least they usually have a discloser that the size may vary a couple of cms (centimeters = 2.54 an inch).
Any one too lazy to provide the measurements doesn't get my business. You order in good faith and receive ill fitting clothing and then have to
suffer the returning postage at your expense. Also beware of sellers that post a stock photo and then trying to do eyeball measurements then
realizing that the stock photo is for a smaller size. With a keen eye one can interpolate and get a close idea of something that fits or not.
Take care and find out what the fabric materials are - Elastane Lycra Spandex can be your friend with stretchable fitting. Clothing items that you
have to fasten with buttons zippers etc. are designed for an exact ridged fit - not much tolerance for a flexible fit. Fortunately swim wear is not
this way.