I have been to Vegas several times with the family. We use it as a gateway to California. Dont go at the weekend (dramatically cheaper rooms) and under no circumstances stay for more than three nights, preferably two. The Mandalay bay is very family friendly with an amazing pool beach with a crashing wave. Also there is shark reef there, an amazing acquarium.
The time difference from the UK is - 8hrs, so its a real bonus to have a 24 hour city to help you time shift. There is an endless selection of things to do for kids and adults without spending a minute in a casino. Shopping is great at many of the resorts and there is a vast outlet mall a short distance away.
Its nasty at the hight if the summer. Way too hot
If you are not on a budget, stay at Aria, the Cosmopolitan, Mandarin Oriental or the Wynne. The best show is La Reve, the best restaurant is Bazzar, the best shooting gallery is The Gun Store, the best pool is at Aria, the best indoor mall is at Venetian, the best cheezy kid thing is Tournament at Camelot. Yhe best out door experience is the fountains at Bellagio.
When you are done their, drive across the Mojave desert to LA and down to Laguna Beach. Turn back up Pacific Coast Highway, past LA and up to Santa Barabara, then up to San Francisco. turn inland to Yosemite, go through the park spending a night at the Awanee and then across the Tioga pass crossing the Sieras. down past Mono Lake to Death Valley, and in one day be back in Vagas. the perfect Californian adventure. I have done variations on this a few times. Kids love it.
Best of luck.