still playing trains ...

Right-o! Last time I got as far as Paris ...

My options in France & Italy are (second class, adult and child saver fares, from the Rail Plus Australia website):

France-Italy Eurail Pass: $A358 (adult) + $A177 (child) for 5 days in 2 months (which would allow for one day trip from Paris, 3 day trips from Rome, and a discounted fare on the overnight Artesia train between Paris and Rome (although there'll be a supplement to pay for a sleeper, and if we buy a second class Eurail Pass we may not be able to book a double sleeper compartment - which is first class. If we can it will still cost between $280 and $500, and I'm not quite sure why such a large price range).

France Eurail Pass: $A242 (adult) + $A121 (child) for the minimum 3-day pass
Italy Eurail Pass: $A210 (adult) + $A106 (child)  for the minimum 3-day pass

Point-to-point tickets:
Paris-Vernon (closest station to Giverny): $A72 return (1 x adult + 1 x child)
Paris metro card (Paris Visite), zones 1-6 (which includes travel to Versailles), 5 day pass: $A103 (adult) + $A52 (child) (there are also 1, 2 and 3 day passes

Artesia overnight train Paris-Rome: $A534-$A656 (1 x adult + 1 x child, double sleeper)

Rome-Pompeii: I've found a huge range of prices, but I think the maximum will be under $200 return for both of us, and quite possibly a bit less than that.
Rome-Orvieto: about $100 return for both of us
Rome-Tivoli: under $50 return for both of us

Another alternative in both Paris and Rome is a 2-day hop-on hop-off pass - very touristy, but will give us a good overview of the main sights, and after that we can do a lot on foot.
Paris: $41/$21
Rome: $26/$14

And I think, now I've worked through it, that's the way we'll go: purchase hop-on hop-off passes for both Paris and Rome, point-to-point tickets for the day trips, and the standard Artesia double sleeper fare. We'll be in Paris for about 5 days, so 2 full touristy days should be more than enough, plus the trip to Giverny and possibly Versailles, and at least one quiet day just wandering on foot. Then 12-13 days in Rome - 2 days with the hop-on hop-off pass getting to know our way around and seeing all the 'must-see' sights, 3 long day trips, and the rest at a slower pace, primarily on foot.

Fabulous! Now I don't need to think about trains again until the beginning of July, when I can make reservations (most of the trains seem to have 90 day limits on booking).