Union is kind of "out in the boondocks" even for those of us out here in McHenry Co.
First, I know of a few places in Union but most are conencted to bars and are NOT kids places. Donley's is an idea but not a fair one. They do have a steakhouse that serves decent food, but ... would you take your kid TO the Wild West without being roped into going into the cheesy attraction. It would be like taking your kid to Disney World ... for dinner.
As for Crystal Lake, I do eat at Round the Clock (Rt 14 and Pingree) and I can assure you thjat the overwhelming majority of their customers are not 80+ and they have pretty good food and pies. What I like about them over their competition is that they serve DIFFERENT and creative dishes in addition to the favorites.
I do not eat at Village Squire (Rt 14) as the food is not as good, the service is not attentive and it is about $2-3 pp more. There food is decent but they aspire to the ordinary.
I avoid the Crystal Palace as the food and service has been bad. At Andy's the food is decent but boring and the service takes forever.
Walker's Pancake House serves good food. It is a bit pricy but it is a place that kids generally love.
Across from the Crystal Lake train station is Georgio's which serves some pretty good pizza.
I have no recommendations in either Hundley or in Marengo. There are a couple places with promise but none that I can heartily recommend at this point in time.
I am assuming that you are coming from the city and wish to stay south of Union. I have a few more remote ideas if you are interested.
Up in Hebron, IL, at the traffic light, there is a place with an old soda fountain that serves breakfast and lunch only. It is across from the dairy whip. They do pretty good ice cream. On the other corner is a pizza place that serves pretty good pizza.
About 0.5 miles north is Crandall's which serves great broasted chicken at a very reasonable price.
Also in the vacinity, is a large apple orchard that has a nice play area for the children and is worth a visit. I believe that it is Royal Oak - Cathy2 wrote a post about it recently (hanging with all the over 80+ crowd seems to be affecting my memory (g)).
I agree with Kim, Checker's is a pretty good place if you get there before 4 pm or so when they fill up for dinner. Also, if they are serving food that day, the old guys running the Union VFW serve up some wicked food.
OP - if you have any questions, let em rip. I am on a road trip to the Rio Grande Valley starting tomorrow .....