I'd probably put the first Percy Jackson in broadly the same scariness league as HP4, although there is an apparent loss of a parent early on that might upset a sensitive child. Probably in general more aimed at 9yo up but I imagine there would be quite a few 7yo who would enjoy them.
I've been looking for ages and bought a boxset but have just caught ten minutes of the film on tv and just wondering whether they are appropriate for a seven year old. He's a great reader but we are quite careful about what he reads. So for example we've let him do the first Harry potter books but nothing after number four since they get a bit dark (I read the scary bit at the end of number four to him).