The most important thing to remember when dressing kids to go to the park is to dress them in layers. As the day goes by, the temperature can rise and drop significantly. Also, active children get warm quickly. If a child is dressed in layers, then he can regulate his body temperature more easily by adding and removing clothing as he feels necessary.
Kids need to be comfortable when they are jumping, running and playing. Sweatsuits are great because they are comfortable and nonbinding, allowing your kid the maneuverability he needs to score a goal, climb the monkey bars or even do somersaults. Jeans are practical too because they are durable and stand up to big bumps and scrapes. Tight jeans, however, might be too constrictive. Shorts are good in the summer, but beware of the exposed knees. Young children should keep from running on gravel or pavement when wearing shorts as a fall could lead to a skinned knee. Avoid skirts for girls unless they are the kind with a pair of shorts sewn into them. Skirts are no fun at the playground because they limit a girls activities due to the fear of showing her underpants.
On the feet, go for trainers. Trainers (or tennis shoes, or sneakers, depending on where you live) offer good foot support, are comfortable, and are flexible enough for participating in all sorts of fun outdoor activities. In the winter, you might want to opt for boots, and in the summer you may choose sandals. Even in summer, however, I do not recommend dressing your child in flip-flops for a trip to the park because they fall off easily, and they are a tripping hazard for younger children.
I think its wise for children to wear a hat at the park, either to keep their heads warm in cool weather, or to protect their scalp and eyes from the sun in the summer months. For girls, it is a good idea to pull her hair up and out of her eyes. It can be dangerous if she is playing a game, or climbing and her visibility is compromised by her hair.
Backpacks are good for storing snacks and small toys in as well as antiseptic wipes. A backpack is also useful if your child is a scavenger who just loves finding small treasures wherever he goes.