You'll find plenty of tips at this website for Keeping Happy On Family Trips. Below are a few cautions about pitfalls to avoid.
Don't Try to Push Too Much Into a Day
This is true whether you're in Paris or in a theme park. Don't try to do too much! Kids feel the pressure right away. The whole flow of the day shrivels if a parent has their eye on the clock and their mind on a schedule, and also a pressured pace precludes those spontaneous encounters that can make wonderful memories.
If sightseeing, schedule one major activity per morning, or afternoon; if all goes well, you can perhaps slip in a second activity. (But don't mention that possible second activity until you're certain there'll be time for it!)
Another point about sightseeing, in, say, Paris: don't take a bucket-list approach, as if this were your one chance in a lifetime to cover a checklist of sightseeing. Instead, this is your one chance in a lifetime to enjoy Paris with your kids age 5 and 7, or 6 or 12, or whatever: no matter how many times you might return to Paris, you will only be there once with kids this age. So make the most of it, and do the things that are most likely to be enjoyed with kids of that age.
Don't Forget Simple Play Time
Take the example of a Disney World vacation... Many a parent has found that kids would rather spend time playing at the resort pool than going to the theme parks, so allow time for that kind of fun. Meanwhile, if you're sightseeing in a city, head to the parks and playgrounds where local kids go.
Don't Forget Down Time
Just as kids need to run around and move, they need time to get calm and get grounded. To again take the example of Disney World: make sure to allow time for breaks from the theme parks. If you have little kids, and the budget allows, stay at one of the resorts near Magic Kingdom so you can easily take a monorail back to your resort for an afternoon rest.
Don't Turn Your Nose Up At an All-Inclusive Resort
Ever heard this phrase? "I never thought I'd go to an all-inclusive resort, but we had a great time with the kids at this place..." Many parents, B.C. (Before Children), consider themselves to be travelers-not-tourists, the types who seek out authentic local places to stay and eschew the gilded mini-worlds of resorts. And yet, with kids, the conveniences of an all inclusive resort are monumental. Kids can help themselves to drinks and snacks all day -- no need to even keep track of how much these refreshments cost. Buffet meals mean no frustrating waits for hungry kids (and sit-down dining is typically available, if wanted.) Kids clubs offer supervised fun for specific age groups and some resorts go all out to offer superb and creative programs -- see Club Med, for instance.) Also kids can easily try new sports and activities: for example, many resorts offer free snorkeling trips, and a young child may only want to to stay a few minutes in the water on his/her first try. No problem; come back again tomorrow. The whole situation is easy, and very different from a one-time excursion paid for a la carte.
Don't Rule Out Alternative Accommodations
When visiting a city, don't limit your search for accommodation to hotels. Airbnb is a fast-growing website that many families use to rent entire apartments for their stay-- perhaps complete with cribs and baby gear. (See AirBnB for family travelers.) If you're heading to Europe, know that many hostels these days offer family rooms and private bathrooms. Forget that old image of "youth hostels"! Hostels are often located in character buildings, and many people like the friendly vibe and chance to meet other travelers. (See Family-Friendly Hostels - Affordable Lodging in Europe).
Don't Spend Your Vacation in Cyberspace
It's all too easy with today's technology for a parent to spend precious vacation time either in his/her work-space or so engaged in online socializing that travelers literally are hardly present in that dream-destination surrounding them. It's a shame for anyone to lose the opportunity to "be here now" and enjoy the beauty of a destination because they're absorbed in a screen just as they could be at home-- but even worse to do this on a trip with little kids, who are so eager to spend real present-moment time with mom and dad. Make the most of those family trips!