We saw it and my 10-year-old daughter loved it, but it likely might bore rather than scare your four-year-old. Whether your older child would like it depends on whether he or she likes more grown-up adventure movies or is really looking for a funny kids' cartoon. Someone else posted that their kid found it dry and boring but it's not supposed to be a humorous movie each minute like "The Muppets" or "Puss in Boots." "Tintin" is much more like "Raiders of the Lost Ark" -- constant motion, constant adventure-high-risk setups, not necessarily funny situations but not particularly scary either. The plot, which isn't much but does keep rushing along, may get totally lost for a seven-year-old.
I think I'd skip it for your kids in favor of "The Muppets" or "Arthur Christmas." The latter is slower and has more kid-friendly stuff in it though some kids might also find it too slow; it's more a British-humor kind of funny, if you know what I mean. If your older child has seen the "Wallace and Gromit" videos or movies, then he or she will like "Arthur," which is by the same makers.