I would recommend usborne every time. We have the big book of dinosaurs as well as the lift-the-flap body book. A slightly greater investment could be a leap tag pen. As well as story books there is an interactive map, solar system pack and yet more body and dinosaur stuff. My ds is just 5 and has loved the books and map.
For Christmas I bought DS a set of ladybird books. This series and the set included whales / dolphins, sharks, castles, pirates, bugs, Space, dinosaurs and deadly creatures, although the list on Amazon is more comprehensive than those. I've been really please with them as a set as they have just enough detail.
got to dash to collect from preschool but we have 2 books about bees and honey. will dig them out later and post back - one is a cat in the hat 'show me the honey'? think thats the name and the other was in a set of little books which are great. will be back this evening with suggestions. I LOVE non fiction stuff for kids.
Dorling Kindersley books are brilliant at that age. They have lots of pictures and photos and are very good at explaining. Ds started interest in facts at about 3yo, and we have found those books particularly good.
Some Usborne ones are good too, and you can also use the internet. Ds was doing his own research as soon as he could. When I heard a cry of "How do you spell "Tornado"?" or something I knew he was researching. It also helped his spelling no end. If you add "Kids facts" after a topic you want to research, then it should come up with more kid friendly stuff.
But don't leave them alone, particularly if they do youtube. I reckon any innocent search is only about 2 clicks away from something you don't want them to see.