For those who don't know, STEM is an abbreviation of Science, Technology, Engineering and Maths. The resources here are things which are usually outside the remit of things that are done in school - although there will be crossover, lots I would hope!
Steve Spangler Science Experiment Library
Steve Spangler is my absolutely favourite Science Presenter (and this is my specialist area as I'm one too!). He has a great presentation style and is fun to watch plus he has a huge library of experiments you can do at home with reliable instructions and clear scientific explanations. Exciting science for children from about 5 to 16 plus adults will want to get in on the action.
There are lots of little Lego challenges online. Here's a good printable I found to get you started with ideas.
"Create stories, games and animations and share with others around the world." Scratch is a well known block coding website made by the famous MIT in American. It is free, quite straight forward to use, easy to set up an account and there are lots of suggested projects online. Your child may also be using it in school and have learned a bit about it already. Suitable for children from about 6 to 18.
Left Brain Craft Brain
The fabulous Anne who is Left Brain Craft Brain creates lots of very exciting resources for crafty engineering. Suitable for kids right up to teens her ideas are clearly presented and use everyday materials you will probably have at home.
Kitchen Science - Science Museum
The Science Museum has produced a pdf of their Kitchen Science booklet. You can print it out or just read the instructions online. 10 great experiments with clear instructions and equipment that is easy to find around the house.
Get experimenting! Sphere Science - a science education consultancy - have released two free booklets of tried and tested experiments for you to do at home with your primary aged children. Download or print off. Click on Activate 1 and Activate 2 in the subtitle of the homepage.