Minecraft is fairly complicated and my 5-year-old doesn’t care much for it. It uses dual analog controls, which my son tends to struggle with. On top of that, there seems to be a lot of tiny text and some complicated menu/inventory system.
On iOS, there are two Toca games that are much simpler for building. One is a side scroller similar to Terraria, one is more 3D like Minecraft. I haven’t watched my son play them much so I can’t comment too much on how good they are, but they do seem simpler than Minecraft.
This is a great question, though, because my son adores building and I’m always looking for a game that will let him build freely. Unfortunately, too many of them are just a tad too advanced. On the PS4 I have Dragon Quest Builders and LEGO Worlds and both are a little too advanced. The LEGO game has been my biggest success but my son struggles a bit with the controls, which are more complicated than other LEGO games.