Where can I find last minute family accommodation in the Baie de Somme (Picardie) in France during peak weekends?

While I can't find one off-hand, this feels very much like other accommodation questions. You're after last minute accommodation with a destination and number of people.

I'd recommend trying Expedia, AirBnb, and consider booking.com and hostelbookers (they do sometimes have guesthouses or private rooms - it won't necessarily be in a dorm).

You could also be pushy and ask the accommodation that cancelled to please find you a suitable replacement, as it was their error. Or if you aren't comfortable being that pushy, consider asking them to suggest a place, or if they can call some places on your behalf that they know may have space.

You could also rent a caravan, or go camping - more likely to get a spot that way and don't need a hotel.

Try to contact the regional tourist office and ask them. As time is short, call them directly and ask them about availabilities. There may also be local offices, which you could contact.

Furthermore, don't be too picky about the region. If there is no availability in the Baie de Somme, widen your scope and look the Côte d'Opale, which is a very nice place as well, but also rather popular during long weekends ...

We did find a last minute camping offering family accommodation afterwards, only to find out that we don't need to have bothered. It is perfectly okay to just go there without any reservation. The place is flooded with b&b and self catering options. Every host seems to be part of some informal network in case they are complete. They just call around and find you alternatives.

We knew this from our pre-kid era, but it works with families as well.