You are exactly right, although it's probably very regional to the American south. =) If a kid told me their mom was going to "give them heck" I'd tell them to turn tail and run! They're likely to get a whupping (eg spanking).
To me it could go either way. I think they're going to be in trouble, but whether it's a scolding or something more physical, I couldn't say just from the wording. However, because the kids are out frog spearing, I'm assuming they live in the country, and knowing country folks I would bet that they at least got their ears pulled or something. I think Southerners as a rule tend to be a little more old-fashioned (read: physical) about disciplining their children.
Give heck is a euphemistic way of saying give hell. If spoken by some children, it might sound normal. If spoken by an adult, I would assume, most times, that they are prissy and prudish. In my experience, it almost always describes a verbal act, but others may have had different experiences with it, especially in other regions of the English speaking world. Decades ago, it (Give 'em hell!", not the watered down "heck" version) was used at times to describe a fierce military attack or rough conduct on the field of play of contact sports.