One year my stepsons' mother got them guitars for Christmas - 1 acoustic and 1 electric and I think they were 8 and 9 at the time. Apparently her intention was to sign them up for lessons so they could actually learn how to play (they didn't play any other instruments and did not know how to read music) but told them she couldn't afford the lessons this year and they would have to wait until next year. Now, I'm sorry, but that's just stupid. If you don't think you can afford lessons for a while, I don't see the point of spending the money on musical instruments now. The boys never got guitar lessons and the guitars just sat and collected dust, but Mom still got be the "cool parent" that year for buying them.
A couple years later, the younger brother wanted a drum set, so of course, she got it for him, but again, he didn't take any lessons and never learned how to actually play them. Apparently, their mom's brother was going to teach him but never did. So again, a waste of money.
I guess if it is your intention to actually sign them up for lessons so they can start to learn how to play, then more power to you. But otherwise they are going to just be some expensive "toys" that will quickly lose their appeal. My advice would be to sign the 6 year old up for guitar lessons but he may need to learn on an acoustic guitar first. Maybe some day you guys will have your own little home-grown rock band!
I grew up taking piano lessons and eventually also played clarinet and saxophone in band. If my daughter was interested in any instrument, I would make sure she got lessons on how to actually play. It really bothered me that my stepsons never had the opportunity to actually learn how to play the guitar and drums - I would have happily paid for lessons myself but their mother was really touchy about it, probably because she knew she herself could not afford it.