I can't offer a recommendation based on direct experience (although I once had a week on Lake Garda 40 yrs ago - kind of like a beach holiday, and close enough for a day trip to Venice).
However for the the type of holiday you've described, Reka might be worth looking at.
They are a Swiss company specialising in value for money, family-orientated holidays in Switzerland and neighbouring countries - and offer beach, self-catering and camping options in France and Italy.
Here is the link to Beach holidays - you can choose from sub-menu (l.h.s.) options, or fill out details in the search area or specify from destinations lower down:
Here are examples of what they have for
Venice:
You can select English as option for their web pages, but information is limited, (specific holiday details in Swiss languages only) but it does give a phone no. if you need to ask anything in English, at the bottom of page.