As a fan of retro games and text adventures, I find Adventuron pretty impressive. The blend of retro style pixel art and simple two word command adventures is pretty appealing and is especially well suited to introducing kids to the genre. I tried a fairly challenging treasure hunt game called Two yesterday and I appreciated how minimalist1 yet fun it was. It reminded me of the classic Scott Adams adventures with a few modern touches. I knew it had mobile support so I tried it again this morning on my phone and was even more impressed.
Excalibur: Sword of Kings is an authorized port of an old ZX Spectrum game that has been updated to include a built in tutorial. I think it would be a more appropriate introduction to this type of game than Two for most kids as it introduces all the text adventure command conventions in a helpful way. There are several other free games on adventuron.itch.io as well, although I haven't tried them yet.
Once you've got a feel for the system Adventuron Classroom is the extensive web-based game making tutorial aimed at kids 8-12, although anyone can use it. Documentation seems really thorough and I think it's something I'm going to show my older kids and play with a bit myself. The games seem to be fairly customizable based on the differences I've seen between Excalibur and Two although I haven't delved into how hard that is to do for an average user.
Unlike a typical Adventuron game it has no graphics and the text and descriptions are mostly limited to two words to surprisingly compelling effect.