Ok, I didn't get a lot done on this piece on this lazy Saturday (I spent a good portion of my day making this unbelievably cute baby Groot) but here's what I did:
And here's what it looks like with the sketch layer hidden:
I typically work in ArtRage for the painted parts, and then export my layered file to Photoshop for the collage elements. Most of the time I will paint the entire image and then add collage elements using the painted elements as a template. For example, I'll paint the daughter's tutu, but then I will probably use that painted shape to cut out a scanned texture and the painted tutu will end up hidden.
I like to start with the most important parts, the characters, and also tackle parts that I know are going to be challenging, like the hands and faces, right away. I get those areas to a level of finish that I'm mostly happy with, then work on the rest of the piece and come back to them later if need be. I can't tell for sure if they're 100% right until I see them in the context of the finished, or almost finished, illustration.
I try to keep different colors on separate layers, with the exception of places where I want to blend colors, like the shading on the necks and the dad's hand. It means I end up with a lot of layers, but it makes it so much easier to make changes without disturbing things that are fine the way they are.
I'll post more on this piece tomorrow.