According to breed descriptions both are good with children but, as with all dogs & cats for that matter, you must socialize them properly around kids and teach your grandchildren proper ways to greet and handle dogs.
As for separation anxiety that can happen to any breed, but most dogs can overcome it with the right training/desensitization.
If you are planning to adopt a shelter dog you can arrange play sessions/walks with the dogs you are interested in and shelter rules permitting bring the children to meet the dogs to see how the dog reacts to and interacts with them before bringing the dog home.
Wheaten Terrier Info
Miniature Poodle Info