"La niña no quiere probarse el vestido."
Translation:The girl doesn't want to try on the dress.
Yes it is reflexive so think of probarse as try ON, whereas probar would simply mean to try (the dress or whatever). I think your meaning would be understood either way but its good to know. :)
Probar+se - probarse is a reflexive verb. It means the action of the verb is applied to the subject.
Explained more here: https://www.fluentu.com/blog/spanish/spanish-reflexive-verbs/
The action of testing to see if something fits uses probarse, as opposed to testing some wine etc.
I wrote "the girl doesn't want to try the dress on" - it is not as 'proper' as the above translation "The girl doesn't want to try on the dress" - but you'll hear it said a lot!