HI SCG
I lived in Preverenges for 3.5 years with my kids (we moved to the US last October). My kids were 8 and 4 when we arrived.
SHOPS, ETC
There's a village bakers/grocers, with an eye-popping range of bread and pastries and a reasonable selection of tins, dried goods, cheeses and fresh veggies, fruits, etc. It's fairly expensive, but not off the scale mad - it's a proper community shop, not just an arty deli.
The butchers is fantastic, but REALLY expensive. Use for special occasions or emergency top-ups only. For example, a kilo of minced beef is around 20-25 CHFs, but they do bring out top-notch chunks of steak and mince it in front of you.
Pharmacy is great, couple of hairdressers, Post Office, etc - all useful stuff. There's also a BP gas station with a mini-market which does bread (they bake throughout the day, so it's fresher than the baker's at day's end) and basic groceries, plus it's also open late evenings and all day Sunday.
We used Dr Sara Taddei in Preverenges with a clinic in Chemin Neuf (just round the corner from the BP gas station). She's very, very friendly, excellent with kids and happy to see them for general, non-pediatrician specific issues like ear infections and stitches, and speaks reasonably good English. We could always get an appointment same day for something fairly urgent, or next day for more general stuff.
TRANSPORT
Assuming, as V says above, they don't mess the bus up (can't see it happening), then the bus runs between Morges/EPFL every 20 mins all day, and 10 mins at rush hours. From Preverenges, you can be in the centre of Lausanne by public transport within 30 mins. I didn't have a car the whole time I was there, and just used the bus or cycled.
LEISURE
There's a gym a mile or so down the road in Denges, but I'm not a gym type and never used it. The beach is fantastic, and the total hub of the community all summer. They have pedalos for hire, windsurfing lessons, little cafes, ice cream stalls, etc, and there are amenities like toilets and beach showers. It's a very, very safe beach for young children, as the sand slopes very gently and there are of course no currents or waves, being a lake. The commune is very good about organising village events for Christmas, August 1st, etc.
SCHOOL
Will your children be going into the local school? It's very, very welcoming - they are used to the EPFL sabbaticalers coming and going, and never seem to sigh and find it all a bother. (I assume you'll be using the standard EPFL apartment in the village square..? - if so, it's a really good location, very central with lots of families around.) I've known a couple of dozen English-speaking kids go into the school there, and they've all had a good experience, been welcomed by the teachers into the class, had no problems making friends with the other kids and so on. The usual pattern for French acquisition is that children should understand French by Christmas, start speaking quite well by Easter and be confident conversationalists by summer - yours might be on a different schedule if they already know some French.
You do know about the crazy school hours, right..?
It's a great location, honestly. It has a very family atmosphere (so does Morges), there's great transport links for Lausanne, Geneva, the airport, etc (which means visitors can be independent when they inevitably want to come and stay), the school is very positive to non-French speakers (every teacher I met spoke English ranging from enough for communication up to totally fluent), the other mums are a very cosmopolitan, welcoming bunch. I'm sure you'll have a lovely time.
*sad now, and missing Preverenges. Will go and look at the beach webcam and remind myself how cold it is compared to AZ*