Please try this code:
var tg = $('#tg');
var treeField = tg.treegrid('options').treeField;
var id = 1;
var tr = tg.treegrid('options').finder.getTr(tg[0],id);
var ck = tr.find('td[field="'+treeField+'"] .tree-checkbox');
ck.addClass('tree-checkbox2')