Wow, well done. Maybe I will do the same for my 6 year old son who loves Lego as well.
On the note of getting them involved, 2 years I help my 13 year old nephew build a billy cart, and my 8 year old daughter an identical one at the same time. We did everything from purchasing the materials (and how to find it and choose good boards) to measure, cut, and assemble. Each child had different abilities so my involvement varied. I often took them through some design decisions and the implications, and they would make the final decision.
I also learnt a lot about my nephew and gained new respect for him because often I asked him if he wanted me to do the process (cut or drill) and it would be faster to get to the result or if he wanted to learn/practice how to do it first and then do it. Every time he said he wanted to do it himself even though it would be slower. I also found him marvelling at the texture of sawdust and other such things, and he has since made some modifications himself.
I needed to remind myself that it was more important that they enjoyed the experience and had pride in the results than a perfect join etc. (But I'd like to think there are a couple of touches in there that show good craftmanship.
My daughter has also enjoyed her version and our signatures are proudly under the finish to remind us of our fantastic time together.
On a side note, when my own father passed away many years ago, only then did I realise his legacy in me shown in my own DIY tendencies and skills coming from being his little assistant all those childhood years as he got me to hold things, or do some of the tasks along the way.
Keep it up!