Paris and Marseille with kids

I really cannot not give you much valuable information for your specific questions.
Food budgets are so personally anyway, like anywhere you can eat cheaply or eat expensively ; same with siteseeing there are low cost ways and high cost ways of getting around and seeing the sites.
In general if you calculate what you would spend in the US in a tourist city it will be similar ; no reason to overthink it or make it complex. Some things cost more, some less ; overall you pay a little more but you don't tip the same so it kind of evens out.

My main reason for posting is we were in Marseille recently and noticed something that surprised me a little in terms of food/dining.
We were hungry at a bad time (not uncommon for American tourists I think) ; had a large breakfast at the hotel so we were not hungry for lunch at the normal hours so waiting until after visiting the Chateuf D'if ; figured we would eat a late lunch / early dinner when we returned.
Upon returning we must have walked past 50 restaurants/cafes/quick eat options and each one of them the was closed for food after lunch until 7 PM. Of course I have run into this elsewhere but never seen anything like this in a city with so many food choices and they all were closed even take away pizza and kebob type places. A few might have opened at 6 PM ; but it was around 4 at the time so 2 or 3 hours made no difference to us. I believe a McDonalds was the lone exception that was open.
The even stranger thing was many of the outdoor cafes and restaurants were quite busy and open at the time ; it was a nice day outside but they only were serving drinks and desserts. Food was not being served.

Anyway make sure you plan you meal times accordingly ; Marseille seems to be old school when it comes to that compared to say Paris.