I heard that sea urchins are big problem in Halkidiki? Is that true? We are going with children to Gerakini and that's not good at all.
Just read about beach safety issues.
In general Halkidiki beaches are safe for children with excellent bathing quality . However, always take a close look at the sea before letting your children in and always keep an eye on them. Sun is always the worst danger not urchins.
enjoy your holidays
I would agree with Sotiris above, about the sun. Take care of this first of all, and I think that any possible sea urchins that could be found, won't be an issue. I've been bathing in Halkidiki for 30 years, and I've never had any problems with these.
Enjoy your holidays :-)