L.L.
Hello L., I know it's been awhile since you posted this question but I'll give it a shot! You probably know a few places by now but just in case; there are lots of fun places to take them. There is a Children's Museum in Fort Worth that is great. Also, Abbey's Playtown in Grand Prairie is REALLY awesome for kids. Not sure is your 6 year old will like it but your 4 year old definitely will. There is an indoor swimming pool called Kirby Creek that is open all year round. So, if you kids like swimming this would be a great idea!
Other things to do:
Six Flags in Arlington
Forth Worth & Dallas Zoo
Dallas World Aqaurium (awesome)
Dave & Busters Arcade (awesome)
Ice skating rink @ The Parks Mall in Arlington
There's so many things to list so I'll just stop there! lol