Unless you use a barrier (like fine netting) to keep your cole crops (brassica family) free of insect pests, it's virtually impossible to keep them off your plants. The best you can do is scout and act on pests you see.
The greatest threat to your crops are caterpillars, the larval stage of a wide variety of moths. Regular scouting and removal is the best way to control first for the eggs and then for the caterpillars. Once the caterpillar pupates and emerges as an adult, it is no longer a threat. Most likely candidates are larvae of cabbage looper, cabbage webworms, armyworms, and diamondback moth caterpillars. One word on caterpillars: they are excellent food for nestlings, so decide how much damage you can accept  and save some food for beleaguered birds.
Other insect pests to watch for are whitefly, aphids, harlequin bugs and flea beetles.
Here are control strategies in order of preference: Cultural control means ensuring you have cleared all weed debris from the garden, and destroyed  all plant matter after garden cleanup. 
Mechanical control involves hand picking caterpillars and eggs or washing off other insects and their eggs. This requires regular scouting to ensure you destroy as many insects as possible. Also, early use of barriers such as fine netting can help keep insects from laying eggs on your plants.
Biological management involves planting a variety of flowering plants to attract beneficial insects that prey on pests.Pay particular attention to flowers such as sweet allysum, cilantro, fennel, dill, buckwheat and parsley. 
Finally, use chemical controls as a last resort. Broad spectrum insecticides kill all insects, good guys and bad. If you must use chemicals, scouting plants will tell you when insecticides would be most effective. Target only larvae and adults. Once pupation starts, insecticides are no longer necessary.