There are two we recommend. The first, Word Search 2 Young Readers Edition by KwiqApps has organized this word search by grade level, and the lowest level begins at Kindergarten. It contains a variety of vocabulary, including Dolch words. The second app is WordSearch Kids HD. This one is designed for younger kids and uses all capital letters, making words easier to find. There are three levels to this one, and the lowest level may be suitable for the youngest speller with short, five letter words.
We have 2 Word Search games for kids. Both are available on iOS and Android. Word Search For Kids and Word Search For Kids 2. You can see them both at http://www.elf-games.com