Once you've found a lot of the letters in the puzzle the best tactic is to look for areas of the grid where there is a lot of blank space. If you don't see any of the missing words, the remaining words in that space are probably reversed or diagonal. Scan for any phrases that match in this area and you're likely to find your remaining words.
A parent or teacher wanting to expose children to word search puzzles could of course buy a printed book full of word search puzzles, but the problem with this is that you don't get to choose the words. An alternative idea is to make and print your own puzzles _ with the help of appropriate word search puzzle maker software it's actually very quick and easy to do. Using the right software also provides other advantages _ you can print as many copies of each puzzle as you want, and you can even choose the size of the letter grid or directions words are hidden in (for example by turning off diagonal and backward directions), so as to adjust the puzzle difficulty.