They're both fine. The first sentence suggests that there are more than five words hidden in the picture and the second suggests that there are only five words hidden in the picture.

Both are correct, but the implication is different. Can you find five hidden words in the picture? This implies that there might be more than five. Can you find the five hidden words in the picture? This makes it clear that there are only five. As an instruction for a puzzle, the second is the more appropriate. The instruction is telling the child to find the five words which the designer has incorporated into the picture.