Both will be very hot - Spain probably hotter. As I kid, I still remember staying in the paradors in Spain & Portugal & the 1st time I visited the Alhambra. Rome is interesting for kids, but unless they are art-freaks, Florence less so. Venice is incredibly exciting & different for kids. Hire a water taxi for an hour w/ an English speaking driver & do a tour of all the back canals, see school kids (well, maybe not in August...), go by the cemetary island - all very interesting & memorable.

I'm assuming you have young ones...