I have a D200, D70s and my wife has a D50, so I feel qualified to answer. For the purposes you stated, you will love the D50. Just but some decent lenses, such as the Nikkor AF-S 18-70 f/3.5-4.5G ED DX lens and not the "kit" lens. She has this lens and I chose it EXPRESSLY for her usual subjects, which include children and landscapes. It has worked out well for her, so it would be a good choice for you. If you just have some money burnign a hole in your pocket, I think I'd consider getting an better lens instead of the D80. First of all, you are probably going to have a hard time finding a D80 at list price for a while, since it is already highly anticipated. I have an 18-200 VR (Vibration reduction) Nikon lens and it's the bomb. It costs about $1,000 (sometimes less), but it might well be the only lens you ever buy for your purposes. That VR really works and you might get some better scenic shots later in the day when you find that you're out there without a tripod.
Another thing to consider... The D70s is being closed out for "only" about $150 more than a D50. The D70s and 18-70 lends I mentioned would be a killer combo for about $1,000.
Getting the right memory card is also important so that you won't spend your time waiting for images to save. Get a 1 GB memory card, such as the Sandisk "Ultra" for decent write speed. You can get these in the $30 range. An "Extreme" is faster, but I doubt anyone would notice the difference except a pro.
To recap: I think your choice of the D50 with the 18-70 lens is an excellent choice for the things you want to photograph. The D80 will cost about $450 more than the D50, if and when you can actually get one.