Ukitake would like to talk to the magic users of the castle and find out if there is a potion or spell to alter ages. Then have a team of fighters take said potion to become children and lure Slenderman out, once they are able to find him / in his realm then take the reverse potion and fight him. And/or give ageing up potion to the kiddies trapped there as well once they got nabbed.

I realise this comment makes no sense.