If you're 21 then you should already have a youthful face. There's really not too much you can do since you don't have wrinkles or age spots or anything like that. Avoid contouring the sides of your face (you can contour the top to make your face look shorter and wider), apply a brighter pink blush to the apples of the cheeks, apply white eyeliner to your lower waterline and in the corners of the eye, and over extend your eyeliner to make your eyes appear bigger. Add circle lenses if you want them to appear really big. You could also do a gradient lip which is popular in Korea and Japan.
Also, lolita is not a child's fashion or age play, so you really shouldn't be trying to look exactly like a child. It's fine to want to look more youthful, but doing serious effects makeup to look like a toddler will look very costume-y, and you already are young! Look at some of the more famous lolita models like RinRin and Misako. Yes they look youthful, but they don't look like children, they just look like young women.
Keep your makeup simple and dewy, not overly matte or powdered. Avoid darker colors and contouring and you'll have a nice, fresh, youthful look.