A hawk cannot really pick up more than 2 to 6 lbs of weight so no a hawk could not pick up a 20 lb dog and take off with it.
A smaller dog or cat a hawk could likely pick them up and take off with it and kill it and eat it but larger animals and larger dogs above 6 lbs of weight the hawk would not be able to lift the animal up.
Hawks can and sometimes will take off with dogs and cats and other small animals for food but most times a hawk will leave the animal alone especially if there is a human with the dog or other animal.
Hawks especially the chicken hawks can pickup and take off with chickens and kill them and eat them.
If you have small dogs and they are outside in a kennel it would be a good idea to put a top over the kennel to keep hawks and chicken hawks out.