1st line has to have 5 syllables
2nd line has to have 7 syllables
3rd line has to have 5 syllables
A syllable is part of a word which has one vowel sound.
El-e-phant has 3 syllables, whereas Chil-dren has only 2. Sun has 1 syllable and was has 1 so does ball.
If you need a bit of practise to understand syllables, then work out how many syllables these words have:
The sun was shining (5)
The children were on the beach (7)
Playing with the ball (5)