Either make it an indoor party, or make it an umbrella party, or a "Wellybration" !!! hahaha, we had one once, summer bbq planned, woke up, pouring with rain...told everyone to come anyway, but bring wellies and rain coats etc... it turned out to be brilliant fun

and anyone who wanted to stay indoors (i.e. grandparents etc) stayed indoors and we opened the patio doors so they didn't miss out, and all the kids loved being allowed to splash about in puddles and pouring rain...it ended in everyone getting a little bit silly and the big kids amongst us having a roll in the mud

hehehe, probably the best bbq we've ever had!!