Vegas is usually pretty easy to 'sneak' in an extra guest as there are plenty of places for the 'others' to window shop or explore parts of the hotel while 2-3 people go to the check-in desk. However, most standard rooms at the mid-tier properties in Vegas that have 2 beds don't necessarily also have a pull-out sofabed (and if the hotel does have rollaway beds, they may not allow them in rooms with 2 beds based upon fire code)... so if you do opt to try and 'sneak in' with 5 occupancy i think there is a good chance the 3 kids will be sharing a bed regardless. Also, keep in mind that most properties charge more for rooms with 2 beds (vs. rooms with 1 bed).
To increase your chances of not encountering any issues, i would suggest booking a PRICELINE EXPRESS / HOTWIRE deal that offers a Room Choice (with one of the choices being a room with 2 beds).
It's fairly typical in Las Vegas for weekend nites to be 2x-4x midweek rates. You may also find this at off-strip hotels, but to a lesser degree. However, keep in mind the time it will take to change properties as it often takes longer than expected (not to mention one hotel's check-out time being a hours before another hotel's check-in time). Do you already have a hotel and/or price in mind for this Saturday nite, should you choose to stay a different property this first nite?
Are you flying in? If so, are you renting a car?
What is your target budget for the 4 nites (excluding taxes, fees, etc)... and although things could change over the course of the next month, i believe Caesar's properties (which includes Bally's and Flamingo) is current not charging parking fees for guests who are staying at the hotel.
Please remember to return to BetterBidding and use the PRICELINE and HOTWIRE links on the board to begin your travel purchases... or thru our sister site MAPSandHOTELS.com for your regular rate purchases.