The refuge is also a great place to fish. It just has some laws specific to it so know those.
This is good information..... I used to compete in the Hagerman kids fishing tournament and they even gave out trophies; since you mentioned grand kids you should check out this tournament if they still have it. Its a lot of fun and if they still do it like they did 20+ years ago, they have a designated large pond that is off limits to public fishing until AFTER the kids tournament.
When it comes to fishing Hagerman though the best advice I can give you is to actually get out a map - and familiarize your self with the boundaries of the refuge and different bodies of water on it..... THEN call up the refuge office and see if you can talk to any of the staff or game wardens that work the areas; mention that your looking for places to take your grand kids fishing and ask for their suggestions and to clarify any rules you need to follow.
Most of the refuge staff(although not all) will really try to point you in the right direction.