Willow tree help

hi! I have a willow tree that is sick, and I want to know if/how I can save it. some of the branches are dying and falling off; some branches aren’t producing as many leaves; there is a black substance on the trunk; but there is also new growth low on the trunk? the new growth gives me hope but the past two years it definitely has not been as healthy.

my 5 year old loves this tree so if I can save it, I’d love to!