Cops better have a legitimate reason to arrest her. They just can’t arrest her because they ran out of leads and can’t think of anyone else to pin it on.
Murder of a 10-month old, 999 times out of 1000, is either the mother or her boyfriend.
This isn’t rocket science.
That there’s a BF is no surprise, and makes mother’s guilt a near-certainty.
Police will often arrest somebody, if they are either wanting to pressure them to confess or to remove them temporarily, so they can’t interfere when evidence is being sought.
Actually, in most jurisdictions, if the police have just cause, they can arrest you and hold you temporarily without being charged with anything.