What to Wear?
Always wear clothes that you will be comfortable in. Make sure that selected clothing flatters your body and you feel confident wearing it. Garments that allow movement are definitely a plus, especially for little children. Think about your location. Ideally, your choice of clothing should stand out against the backdrop of the selected location.
The key to looking timeless as a family or couple is to coordinate your clothing choices. You should complement each other in terms of color - and selecting a color theme is one of the best ways to accomplish this easily. Go with 2-3 colors and offset them with a few neutrals. Each family member should carry through their own style and really express their personality with their clothing choices. I find it easiest to achieve a coordinated look if I start with one patterned/print piece and then pull coordinating colors from it.
Layers, Accessories & Textures
Layering and using textures are very important. Adding a scarf, a knit cardigan, hat, or stylish jewelry is one of the best ways to add color, individuality, and interest to your portrait. Textures in particular add an incredible amount of visual interest to pictures and especially look great in black and white.
Patterns Are OK!
There's nothing wrong with wearing patterns and stripes. Just make sure they go with the color theme you select and include plenty of neutrals to carry them through. Pair small patterns or big stripes with some textured pieces and you're set!