Having them take the train is an excellent solution - not only is it economical but also convenient (no changeovers)
However, if they want to take a taxi and you are driving up the Siena - Firenze Raccordo (the main road that connects these two cities) you will find a taxi stand after the round about heading towards Galuzzo. There is a large parking area on the right hand side, bus stop and taxi stand. BUT - you should probably have a number ready to call (055 4242 or 055 4390, because taxis are not always there waiting. When you call you can give them the location (Via Cassia, 20-22, 50023 Firenze FI)and they will give you a number in confirmation and pick you up.
You can proceed up along this road for almost as far as you want, the ZTL zone actually starts once you see a large
stone gate and wall (pretty hard to miss
This position near Galuzzo leaves you far enough out of the city center to worry about traffic and close enough to catch the main highways and start touring other great places in Tuscany.