1. Definitely take the vista dome from Tambo/Sacred Valley to MP. I wouldn't do the HB train, and during March it may not even go back all the way Cusco. You can get the train tickets online easily.

2. We stayed at the Inkaterra rather than the Sanctuary. It's definitely an older crowd at the latter, and I enjoyed the Inkaterra ethos more. But they both are a bit of a tour bus type of place and the food at Inkaterra wasn't very good. Having said that, since we were with a 4 year old it suited our needs.

3. I wasn't a fan of Cusco and we stayed at Palacio (and got an upgrade so that our son could have some space). It was fine, the breakfast was excellent. But I guess we just didn't enjoy it a whole lot and liked the excursions from Tambo, and saw a lot of the things like Ollyntambo, Salt Mines, etc... which were very interesting.

4. The Tambo excursions were excellent, and perhaps we were lucky since, like almost all the guides we've had (like Amankora, safaris, etc...) they see young children as a breathe of fresh air at these places.

If I were to do Peru I'd opt for something in the Amazon + sacred Valley (Cusco stuff can be done, sort of) + Inkaterra MP + Hotel B for great food. Tambo was good, but the furnishings were super basic and a little cheap. I'd probably regret not seeing Cusco if I did the above, but I think it would have fit my interests most.

LIM > PEM (reservica Amazonia) > short flight to Cusco > trip down to valley (45 min car ride that you arrange with hotel) > vista dome > MP > train (not HB!) + car to Cusco > Lima to enjoy some fabulous food.

I'd probably contact Inkaterra and book through them as they have a spot in the Urubamba that looks more interesting to me than Tambo.