Good suggestions from the above posters!!
I recently was asked to do a project with ties by someone ... and said NO! Friends were with me and later shared they thought I should have accepted the offer. I said, no ... have never done ties before, and could only see problems with this guy and never being happy with what I did.
Do you really want the other kids to have one?
Wouldn't it be nice for your grandson to have something unique?
I much prefer to make things on my own ... and not be under the pressure to live up to expectations of others. If they like my work, great!!! And they can appreciate it all the more.
If you wanted to stall ..... tell them you are busy now, and if they contact you after Christmas you "might" consider it. That'll give some time to see if they are serious or not. Plus perhaps in the meantime you could make another and see what realistically the costs would be, plus what extra you would like add on to sell it at.