Ellefson Park offers a small turf area, children's play area and a picnic shelter with grills. In the Winter time, there is a designated sledding hill that locals enjoy.
We visited Ellefson Park while in Vail for the holidays. It's great for little kids especially if you have your own sleds and want to give kids a taste of sledding before shelling out $25 at Adventure Ridge's Tubing area. It wasn't crowded at all just us and two other families. There are only 4 parking spots, so I would aim to go early. There are bathrooms on-site since Ellefson is a public park, but they were locked when we were there. There are picnic tables if you want to setup hot cocoa or food.
There's a long flat "catch" area at the bottom of you or your kids pick up too much speed. Don't worry, the bottom of the hill is far enough away from the street.