There are numerous kid trails and activities available. The Yellowstone Institute offers summer courses. Especially light sweaters, wind and water-resistant jackets, and good walking footwear. Most are at boiling point, which is 199 degrees at this altitude. Stay on the boardwalks at all times.
Wildlife is also potentially dangerous.
Do not approach wildlife or attempt to feed smaller critters. Climatic conditions in Yellowstone are always extreme. Always be prepared for the worst. Even bright, sunny summer days can turn into cold, windy rain and sleet storms. Search for stores that sell or rent gear near you.