Mazwad,
My son married a girl with 3 children (from 2 different fathers). I have included them on my tree as grandchildren. My daughter in law explained after she saw the tree, that the eldest had a different surname to the other two, and would n't want her real name on there. I asked this child what she thought, after getting permission from her mother, and the child (all of 11) said "But that is who I am, so my real name should be there".
Maybe, if you talk to your d.i.l. and the boys, they may like to be included as, after all, they are family! Only for your personal use, of course, I wouldn't put it on a public website.
Good luck, JB