Brown marks on the eye teeth?
Cavities on eye teeth are really really uncommon. Cavities are more common in the occlusal surfaces of molars, and if the baby teeth are crowded, *maybe* between.
Are the surfaces of her eye teeth very smooth where the spots are? or is there an indentation?
I wouldn't think it is a cavity, but would suspect one of two things. Staining... is she eating any solids yet? You didn't say if she is or isn't. It is possible. And the other is intrinsic stains, meaning a discoloration that is permanent caused during formation. If you were sick at all during pregnancy, she may have some stains, this normally appears as white rings on the teeth, but can be other things.
A dentist can take a look, eye teeth are easy to see, she couls sit with you and he can peek.
Chances are it is nothing.
When to bring to the dentist? I tried with mine at 3 and they weren't ready, yet my mom started us at 18 months. So it depends on the child. If you have concerns about it, bring her, if you notice changes especially. Those eyeteeth should be in there till at least 10 years old, oral health is very important.