Younger children are most likely to be interested in something which is very hands-on, and gives results quickly. They probably won’t be interested in growing a crystal, and are probably a bit young for something like a robot building kit. Young kids will be most likely to enjoy a gross science kit, or a volcano kit. Also think about safety features when buying for younger children. Make sure they won’t eat anything they shouldn’t, or there are no small parts which could be dangerous.
Be careful: The water only has the power to make the sugar crystals “invisible” when it’s piping hot. After the water cools down and evaporates, the sugar turns back into a solid. And with a little help of your sugar-soaked string, the crystals will find a home to grow upon and become rock candy. Learn how to make your smart sweets with these instructions from the Exploratorium. 
We have categorized by age to make things really easy when deciding which type of experiment kit to go for and start experimenting with. There is everything from geology rock experiments, ocean discovery kits, volcano boxed experiments, butterfly and insect garden kits, robotics activities and kits, human biology box sets and chemistry sets that bring the obscure into light making learning loads of fun and super exciting for everyone involved.  These mentioned are only in the 3 years old and over a section, but we also have a 6 years old and above, because we understand the capabilities of children. The section for 8 years old and above come under one section to help make a clear distinction.
Ah, slime. It’s the one thing that parents across the UK have been being pestered to make, so why not turn it into a science activity? For our concoction, we mixed shaving foam, PVA glue and a little bit of air freshener (fabric conditioner can work too!), to create a slime that you can swirl, stretch and crack, providing hours of fun. Try changing the quantities to see how the mixture changes, and ask the kids to explain the differences.