How can I reduce the chances of my parents knowing I had lip injections?

Do not let anyone tell you that they can guarantee you will not have bruising and swelling after lip injection. This is not the reality. You could poke your lips with a needle or cannula, not inject anything, and you would still have swelling and potential for bruising.  The best way to guarantee they will not notice is to stay away from them after injections, utilize ice, and Arnica maybe worth a try as well. To ensure you are receiving the highest level of care, seek out a modernly trained, new-school dermatologic surgeon, oculoplastic surgeon, facial plastic surgeon or plastic surgeon who is board certified and fellowship trained in one of these "core four" cosmetic specialties.

Cameron Chesnut#realself500 Physician

It's hard to say if they will notice or not.  There is always a possibility of bruising after the procedure as well as some swelling.