My daughter does junior motorcross and I do worry about her a bit with the heat.  When you are padded to the nines with helmet and goggles, boots, padding in every possible place you can pad them, it is easy to get too hot.  Fortunately they only race in the cooler months from April through to November but the Australian sun is rather unforgiving I’m afraid, even in the spring and Autumn.  I try not to put her helmet and goggles on until the last minute when she goes onto the start line and I encourage her to drink heaps of cordial to keep her fluids up and sugar levels too.  I haven’t needed to stress dose yet but I do watch her very carefully.