we’re in the process of buying a new cello for the middle child, this one:
She’s of course a bit bigger now, and needs a full size instrument. Gaad, this is expensive stuff! We’re planning to buy an instrument she can use her whole life, it doesn’t make sense not to buy a cheapo cello, especially since it’s so much more fun to play on a nice sounding instrument. All the blood, sweat and tears sort of make much more sense this way…
computernerd composer 
I’m looking forward to the kids getting into music. Buying the instruments scares the heck out of me. I’m really hoping one will take to one of my Father’s guitars and the other will want my flute.
-Turkish Prawn
Don’t pick cello or double bass, don’t pick tuba, well, basically just ask them: Would you like to learn the flute/guitar? ;)
Then, you can get fairly decent clarinets and saxophones on the net, without having to pay the whites out of your eyes. Probably more instruments too.
You don’t need to get a great instrument, but a good one. I had a decent trombone as a kid, but took up guitar at about age 13. After forty-some years I have very nice ones. I have found a lot of comfort and pleasure in music and will always be grateful to my parents for the opportunity. Your kids will love you for it, if not now, then someday.
Jon, a great instrument is way out of our price range at the moment, anyway. This one’s very nice, if she decides to be a professional cellist later on she might need a better one, but this one’s fine for amateur playing, her whole life, if she wants. :)