Follow the local building codes for how much space to leave between vertical deck rail slats. Gaps that are too large can be dangerous for a child who tries to slip his head through, or perhaps even his whole body. Smooth-sided slats are better than spindle slats. It is easier to climb a spindle slat and fall over the top edge.
Hang a gate leading from the deck to any stairs. Or, substitute with a temporary child safety gate.
Position a grill or fire pit away from the traffic lane, and high enough to keep it out of the reach of younger children.
Keep the cover on any swimming pool if the deck connects to one. Have a motion monitor in the pool just in case.
Avoid using the deck to store hazardous yard materials such as fertilizers and herbicides. Remove any yard tools such as hedge clippers, trowels and rakes.
Consider laying an all-weather carpet on a wooden deck. It makes breaking glasses less likely when dropped and will cushion a minor fall.
Grow only safe container plants on the deck. Some plants, such as hydrangea, poinsettias and amaryllis are dangerous to children and pets alike.
Place shelves and other objects with sharp edges and points above a child's height to keep her from hitting them with her head.
Sand and repaint the deck as needed to prevent splintering and rough spots. Walk the deck and look for where nails have popped out of place and drive them back into the wood.