My 18 year old daughter has become increasingly physically abusive to me and she's had a two year old son. If I were to call 911 on her would child protective services become involved?

Is she living with you? Why does she lash out? If she's not hurting her child, the police might not report it to CPS, although the child does seem to be living in an unsafe environment....

Yes she and her son live with my husband and myself. She gets violent when I won't up her plans to con her dad out of cash for her drug habit

Yes she and her son live with my husband and myself. She gets violent when I won't up her plans to con her dad out of cash for her drug habit
Yup, unsafe environment AND druggie mother. Has she ever gotten help?

Yes, if she is physically abusive you call the police when it happens. CPS may or may not be involved,

See if you can get her help before you have to call 911, or one of you gets hurt.
Every town or city has at least one social worker. Find out who it is - start with the blue pages of the phone book.
You need someone who can help you make a list of rules and demands so that your daughter doesn't keep this up and so that your husband stops enabling her. And so that you can learn ways to be firm with her too. A drug addict who has reached the stage of physical abuse needs not just tough love, but just plain tough. Yes you can throw her out on the street if she doesn't get help. You SHOULD call CPS and they probably will remove her child, and you can apply to take care of him, and get a RO to keep her away until she gets into rehab.
And no, you don't have to feel guilty about any of this. You may have to force yourself to not feel guilty, as a huge part of stopping this enabling. For THEIR SAKE! Get tough for THEM!

An addict reaches a stage where nothing and no one matters anymore. She is not your daughter or even human when she needs drugs. Eventually she will cause serious harm to you, her son, or herself, or end up dead on the street. You can't use reason, bribery, or the past. You DEMAND that she do this or she faces the CONSEQUENCES, end of story, and you don't back down.