Turnips are great in soups or stews, use them like potatoes. Since the taste is much stronger though, I would suggest substituting turnips for 1/2 the potatoes or carrots in a recipes or just adding them. You can also boil and mash them alone or with potatoes.
Yes, they are fine for a 20 month old but you may want to mix them with carrots, potatoes or sweet potatoes if your son doesn't like the taste.
You can also roast them in the oven. We like to use a bunch or different kinds as follows: Peel and cube potatoes, turnips, carrots. Toss with olive oil, salt and pepper, and maybe some chopped herbs like thyme, rosemary, sage - whatever. Bake at 375 on oiled baking sheet until tender (45-60 minutes). Really yummy!