"Her baby often puts its fingers into its mouth."

Translation: Ŝia bebo ofte metas siajn fingrojn en la buŝon.

[Druif]

Is this also allowed: "Ŝia bebo ofte metas siajn fingrojn en sian buŝon." (a second "sian" instead of "la")?

[vikungen]

Yes.

[Lohnesinpr]

Why is it its mouth and not the mouth

[PatriciaJH]

Because "la" automatically means his/hers/its for body parts and family members (takes the possessive.) It's like Spanish. If there we had "la patro" or "la fratino" or "la praavo" those would all belong to the baby too.

[thepepisin]

Why not "Sxia bebo ofte metas gxiajn fingrojn en la busxon"?

[BGXCB]

Are siajn and la interchangable, or must they be used in that order?

[salivanto]

I would have said "la" in both cases.

I recently answered a similar question on Stack Exchange.

http://esperanto.stackexchange.com/questions/3016/%C4%88u-oni-uzu-la-a%C5%AD-mia-kiam-oni-parolas-pri-siaj-propraj-korpopartoj

[KingLaSerna]

Why is busxo accusative

[salivanto]

Motion towards location.

I've written a complete explanation in a recent blog post.

https://blogs.transparent.com/esperanto/keys-to-understanding-esperanto-prepositions/