In raising rats I ve never had an aggressive one. They actually seem to enjoy human company. They spend more of the day awake than gerbils and hamsters do. They seem to be more relaxed and slower moving, which makes it easier for a younger child to handle them. Rats are clean animals as long as they are provided with good clean bedding.
Grooming is a big part of their day.
Rats can be a lot of fun for children. Your child can easily train his or her rat to do simple tricks, or come to them on signal. Their gentle nature is probably their best selling point. Especially when it comes to younger children, a rat would be my choice. With small children always remember that animals can bite.