Here's all I can think of off the top of my head for kids to do, including the places' websites:
Scandia in Ontario is small amusement park w/ rides, arcade games & miniature golfing:
Scandia Ontario Home
Gameworks at the Ontario Mills Mall has more unique type arcade games & things the kids can actually ride on plus a restaurant:
Welcome To Gameworks
A unique restaurant that kids will love is Rainforest Cafe. That's also at the Ontario Mills Mall:
Rainforest Cafe®
Planes of Fame Air Museum in Chino:
Planes of Fame Air Museum - Official Site
Brunswick Deer Creek Lanes bowling alley on Haven Ave:
Brunswick Zone Home (Center) - Brunswick Bowling
Catch a baseball game at the epicenter in RC:
Rancho Cucamonga Epicenter
Ice skate here:
Directions
OR
Figure Skating - Ontario, CA - Ontario Ice Skating Center
Roller skate here:
Home
Maybe there's a play/musical that intrests them at the Lewis Family Playhouse in RC at the Victoria Gardens Mall:
Lewis Family Playhouse
I don't know what dates the kids will be coming, but the Ringling Bros. & Barnum & Bailey Circus will be at the Citizen's Business Bank Arena in Ontario:
Citizens Business Bank Arena :: Citizens Business Bank Arena
You can see what's playing at this dinner theatre in Claremont:
Ben Bollinger's Candlight Pavilion Dinner Theater
If the kids like pizza, there's...
John's Incredible Pizza Co. is an all you can eat buffet w/ arcades, little coin-operasted rides, etc.:
John's Incredible Pizza Co.
OR if you prefer Chuck E. Cheese, there's one in RC on Foothill:
Chuck E. Cheese
Then, a little later are:
July 10 - Aug 9 is the fair in orange county about 40 min away:
2009 OC Super Fair
In Sept, the fair starts in Pomona:
2009 L.A. County Fair - Sept 5 - Oct 4, 2009 -
Otherwise, there's more stuff in Los Angeles, such as...
the La Brea Tar Pits:
Page Museum - La Brea Tar Pits
California Science Center:
California Science Center
The Childrens Museum is being renovated & isn't yet open to the public:
The Children's Museum of Los Angeles