Tree sap is such an awful sticky mess to deal with, especially on clothing. Grandpa is forever getting it on his hands and grumbling but I just give him a dose of Dawn on his hands and get him to wash under cold water and it removes it from his hands.
Because it's so sticky, like chewing gum, I'd suggest you try using some ice cubes to rub over the goop at first and a stiff toothbrush to try to at least remove or loosen the major part of the sap. Of course there will be a stain under the goop so I'd first try pouring some Dawn directly on the stain and brush the area with a stiff tooth brush, if that doesn't work than I'd resort to using White Distilled Vinegar, the acid should eat up the sap but not your jeans and it won't discolor them either. Pour it directly on the stain and again brush it with a stiff tooth brush. If all else fails and you just love these jeans the last resort would be to sew a cute patch over the area.
