Goldfish and lizards can be beautiful and interesting to kids, but they don't love people, and rodents tend to stink and attract other rodents. All pets need to be fed and cleaned and have their needs met.
Pets can be wonderful for kids, and loving a dog, cat, or even a horse will help kids develop relationships and accept responsibility, but having one can be a lot of work and licenses, vet bills, food, etc. all cost money. In my opinion they're worth it, but sometimes it's best to wait until the children are old enough to accept their share of responsibilities before getting one.
What was your first pet and how old were you when you got it?