There are a myriad of possible reasons for her behavior. Unfortunately, there may not be any way of finding out because the mere effort to find out would make her aware that her reasons may be "wrong" or "bad," which in and of itself could cause her emotional stress.

Some possible reasons:

She is an early bloomer and knows exactly what she is doing.

She is regressing and takes comfort in thumb-sucking behavior and being loved like she sees little girls loved, either in real life, in her own memory of years ago, or on TV.

She is mimicking or trying to mimic behavior of others, perhaps younger girls, perhaps older women, in a bid to fit in or do what she thinks she is supposed to do.

She is mimicking or trying to mimic behavior of others, perhaps younger girls, perhaps older women, in a bid to stand out for attention.

Notice that some of those are mutually exclusive.

My advice:
Either do the safe thing and do nothing, or do the pro-active thing and talk to the parents and let them know that you will support them in any way they want to raise their child. Ask them if they want you to discourage, allow, or outright encourage this behavior, then follow through. The one thing you don't want to do is tick off the parents. The other thing you don't want to do is do anything that would give a third party any reason to think poorly of this girl or of you, unless it is a side-effect of the parent's child-rearing and, on balance, the parent's child-rearing is a net positive for the child. Telling a worried third party "it's okay, I've talked to her dad about it" generally calms other adults down.