I am so sorry for your loss!
My best friend just lost her husband 8 weeks ago and she bought a locket for her 3 year old to have a picture of her and daddy in it. She loves it and she decides when she wants to wear it. So if there are any girls that might be something they would like.
Encourage everyone to keep talking about your sister-in-law so the kids have memories and so they know it's ok to still talk about mommy.
I agree with everyone about the memory book. I am doing one for my best friend and the kids. I had a notecards at the Celebration of Life that told people what I was doing and to please send me their memories and/or photos (that way you have stories from a variety of people throughout her life) You can also have some blank 3x5 index cards for people to write a note at that moment so you don't have to wait to collect them.
Hope this helps. Good luck!