It's very very easy to do you own holiday, the hardest bit for me was doing the research to find the best resort for what we were after.
You are spot on, very few French do package ski holidays, the only significant market there is Club Med up-market, just like your Mark Warner and Ski Famille. Many French do self-catering apartments when they have children, often is quite small apartments too (you get m 3 with descriptions). There is a multitude of companies to choose from.
www.goealia.com
www.odalys.com
www.maeva.com
pv-holidays.com
This is just a selection of them. A family of 5 could get an apartment for as little as ?300 a week.
French school holidays are quite significant in terms of how busy a resort will be and how costly lessons and accommodation will be. The most cost-effective week for UK school holidays this year is the week inculding Christmas and the first week of Easter.
Many of the resorts well-known to Brits are also the most expensive. There are tons of resorts where you can go a whole week with hearing only a few British accents in the resort, to keep costs down, this is where to be.
For travel, either drive or fly using a low-cost airline, such as Easyjet into Geneva. Flights from ?50 return. Transfers cost from ?50 return. Another option is the train, connecting in Lille is best with small children as Paris involves across town station transfer. You could go to the Chamonix valley, valley with la Plagne, Maurienne valley. You would pay for just 2 adults and 1 child, costs could be as little as ?400 return from Lille.
Cheapest ski school is always ESF, combined often with a "garderie" nursery either next to the under 6s slope or nearby. ESF ski school starts at ?90 for 6 days of 2-3 hour group lessons. Garderie starts at around ?3 for an hour. This is where many Brits have the greatest problem, where the language and cultural barriers combine and many are unable to cope. For childcare, your alternative is a local English speaking babysitter, you will find an agency for these recently posted details on mumsnet. Or you alternates child care between parents, one skis morning, the other afternoon. For the ski school, there are ESI ones too, add an extra 50%ish upwards to ESF prices, because of extra cost, French are unlikely to ever use these ski schools, means numbers are kept down, more likely to find an English speaking instructor.
Finally. www.peakretreats.co.uk do self-drive and accommodation only and their website is an excellent research tool. Ususally once you have a shortlist of accommodation, you can find cheaper prices for your choices by booking direct eg they market Goelia, PV, MGM, Odalys apartments, sometimes their special offers makes their prices cheaper.
You should be able to put together a week of skiing in France, lifts passes, ski hire, transport, accommodation, ski school and garderie, first week of Easter, all for easily less than ?1600 - 400 per person (1 year old free for everything except child care). We manage it all for ?350 per head and have never gone over that budget.