Hi Iapetus,
I was in Venice last year the exact week you are planning on going this year. It was a bit hot but still fun. One thing I noticed is that we saw a lot of families dragging suitcases a long, long way, dripping with sweat, trying to find and going to apartments/air bnbs. Some had local Italian contacts from the air/bnb followed by the family with children dragging suitcases. Some families were dragging suitcases in the heat desperately looking for their apartments. There are thousands of apartments in Venice. I'd map out your apartment with care, especially with 8 year olds. I'd map out how far from transportation and how many bridges with steps to cross with suitcases.
Originally we planned to get an apartment but Perche said be very cautious when renting apartments or even hotels in Venice. The photos are not what they seem to be. We ended up booking a hotel that had a generous breakfast (more like a German breakfast) and then at the suggestion of folks on this forum we
made reservations to have our big meal in the middle of the day. This worked out really well and due to the heat it was nice to have a break in the middle
of the day when we could sit down leisurely and get a break from the heat. In the evening we had light snacks at the suggestion of folks on the forum.
My boyfriend had never been to Venice and I told him we would be staying clear of St Marks Square and Rialto due to the crowds. There was still plenty
to see and do. We ended up walking through St Marks at about 8:00 pm one night (and things were not too bad) and walked through Rialto to get across the bridge
at one point. Rialto area was very crowded.
The biggest challenge was just staying hydrated due to the heat. I remember drinking a lot of water during our mid day restaurant meal and buying a lot of water
bottles throughout the day. I'd recommend linen skirts/dresses for the women in your party to wear as these were great for staying cool.
We had a great time!