a swollen lymph
node is to say that body is trying to fight an infection.. defence is being prepared in the body to fight the invader whether bacteria virus or other.. depending on the site of lymph node that is palpable the close by site of infection is to be checked for..
in children usually the nodes in the neck are palpable without any apparent cause.. could be due to minor illnesses or frequent introduction of new agents into child's body from the environment.. in majority the nodes are not painful.. and if frequently palpated can turn tender too..
but in your grandson you say that he passes urine frequently, suddenly.. there is a possibility of an urinary tract infection
which is better to be investigated and ruled out before a full fledged infection occurs.. so better to take him to a pediatrician
for the same and get necessary analysis done..