Children tend not to say things that are not true in this regard. If the child says it has a sore leg it probably has. You need to try and get to the bottom of what the child means. It may be that it is a different ailment each time

and it is described the same way as young ones do not have a full grasp of the language yet. A grazed knee and a cut toe could both be described as a sore leg. If the child has a different explanation and it is a consistent explanantion, Eg my leg is sore here and they point to the same spot on their left shin for a week and it doesnt seem to be getting better and there is no obvious reason as to why it would be sore (ie regularly bumping that part of the leg on their bike whilst learning to ride etc) then you should have it investigated. Whilst some may say that you are being over cautious ask yourself this, If you had a pain in your left shin, in the same spot, it has no reason for being there as far as you can remember ( severely drunk on a Saturday night could be a possible reason) and it is not getting better after a period of two weeks what would you do ?  Your child cannot make the decision for investigation or not, you need to do it for them.

I would explore the possibilities by asking different questions and see if you can identify if it is the same ailment or a series of different day to day happenings in a busy life.(Most likely)  If it is the same ailment and it is not easily explainined, (blisters due to new shoes, learning to ride bikes etc) then I would get it investigated.