There was a time when getting kids really educational toys was something that every family did. Each year around Christmas or on my birthday I could always expect presents of books and science toys of some kind. It's rare to see kids getting presents like that now - even though they'd have great fun with them and learn so much.
So what actually makes a science toy a science toy? Anything that leads a kid to discovering new for the first time or seeing a law of nature of the Universe play itself out in front of them would be a good way to define it. This type of tends to make kids more curious about other aspects of science and nature so you'll educating them without them even knowing it. This is something you really want to encourage with your kids - the idea of active learning. Making their learning interesting is a huge key to success.
So what kind of science toys are there to choose from? You can start with something simple like a slinky spring or a basic chemistry set (child safe), then you have the option of a junior microscope or a robotics kit. If they're more biology inclined kids then you could get them an anatomy kit. Or how about an ant farm? Or one of those volcanoes that actually erupts. The really big one is telescope for the little star gazer in your family. I still have memories of sitting in our back yard when I was a kid staring up at the moon through my dads telescope.