One helpful tip I read elsewhere on the forums here was "This and these have the T's": este, esta, and esto all mean "this," depending on the word's gender, and estos and estas mean "these." Ese, esa, eso, esos, and esas are "that" and "those," they don't have T's. I hope that helps some, it has certainly helped me!
In general, as mention above, chica refers to an older girl, while niña refers to a little girl. If you want to use chica for a little girl use diminutive: chiquilla, chiquela, chiquitina or chiquitin if it is a little boy.
An old lady can still refer to her 40 year-old daugther as "mi niña". Sometimes even twenty-something year old girls are referred as "niñas".
"Bonito" may be the best vocabulary choice for a Spanish description of this bike, but in English, "nice," "pretty," and "sweet" could all be used to convey the same idea. It's kind of the same thing as native Spanish speakers starting a conversation with "Bueno. ..." used to mean "Well ,,, ."