They won't smother your daffs. When they come up and bloom in the spring, they are small. It's not until after they bloom that they begin to get big and by that time the daffodil foliage is pretty much done. Depending on the type of violet, those pretty blooms may not be the source of the seeds. The ones that produce the seeds may be at the base of the plant, nearly invisible and hidden from sight. I often rip out old plants and let a few new seedlings come up to take their places, otherwise I would be smothered in foot-tall violet foliage. Violets will make a dense ground cover if you let them, they can be very invasive if you don't keep them under control.

Thanks for your answer. I have noticed since that the leaves and stems are yellowing the way my daffys foliage does - is this a normal cycle for them or could they just be with too much sun/ too little water?