Don't know who all these guests are but I think your instincts are the best since you know your children. As to the argument that you shouldn't push your children, I think that there is a way to be supportive and encouraging and also insisting that there is a time to do it and not let it go. You can make it fun. I was a musical rebel, (my mother was a piano teacher) and I was made to practice and ..........have made my living at music for over 50 years and have had fun doing it. I rebelled and took up the guitar and got into folk music.
Another musical rebel was Yehudi Menuhin who was forced to practice and by his own admission decided that he hated violin and took up the harpsichord for a while. Then he went back to the violin which in turned out that he really loved and did some interesting recordings with Shankar and others. I think it all depends upon how you do it. If there is a strong negative reaction, that's one thing but kids at that age really don't know what they want and should be exposed to a lot of options. Music is one of them. Consistency and modeling are all a parent can do.
As to the two-tuning thing, I maintain that kids can do it. They don't have the hang-ups.
The wonderful thing about guitar is that it can be a social instrument. You can't lug a piano around to a party. But you can sit with a group of people and sing and share songs. It's an ideal home-schooling tool in my opinion.