True Bug Nymphs

You are quite right, they are true bugs. They look like Shield Bug nymphs as they show many of their chatacteristics, especially the antennae which appear to have 5 segments (as you will know better than me since you are in Greece, that's where the name Pentatomidae comes from - penta means 5). So, you might want to have a look on the internet for the shield bugs (Pentatomids) in your area. In the meantime, if you want to, you could change your common name to "Shield Bug Nymphs" and your scientific name to "Unidentified Pentatomidae". Then it will be just a short step to a complete identification. Very nice spotting, by the way!