If you have a digital camera, try taking a photo at 08:00 11:00 13:00 15:00 17:00 19:00 and watch which area is in shadow.
You can also lay down the hose pipe or use random objects from the garden (spade, wheelbarrow, tarpaulin, a chair dragged out from inside etc.) to see what shrubs / trees / lawn / water might look like positioned in different spaces. Take more photos as you place and move all these things about and you can then compare photos and notice what lines / curves look best etc.
If you go back through old photos of people eating / playing in the garden at different times of the year you might also see recurrent patterns which show where the sun will be...
The sun is higher in the sky during the Summer months, and much lower in Winter, so this also comes into play...
Blocking full view of the entire garden and creating the feel of different spaces can be one useful trick, as can creating glimpses of things in the distance by framing using hard landscaping features (arches / pergola etc.) or planting.
Something the pros also do is to use the proportions and style of the house to influence the design: formal vs. informal etc. Also the recurrent widths of blocks of windows can help determine the repetition of scale elements in the garden and suggest a positioning grid which can be a good starter. Our house is smallish, so for example, I needed to keep the individual elements (patio, lawn, deck) in proportion to each other and the house, IYSWIM.
A decent sized Summerhouse / play cabin / shed will help keep clutter out of the main garden area and give kids a fantastic, inspirational play area. You can get them built (at a price) to your own design, or dress up a cheapy from B&Q etc.
A boggy area (whether shallow pond or just wet soil) will attract bugs and frogs and birds, and if that's your thing you'll be in heaven. I made thee bog areas - two for plants that like to be damp / wet, and one for insect-catching carnivorous plants ! Also dug a small pond, part-covered by a cantilevered deck, with a false bottom so the deeper water (needed to support wildlife) is hidden from the little monsters. Investigate the pond options carefully, weighing up your attitude to risk and your knowledge of the kids' behaviour - perhaps consider fencing it off until they can be trusted around water.
I found a book by Robin Williams - 'The Garden Planner' was amazing for lots of really well worked out illustrated designs and good text.