Son would be more appropriate. Ex.¿Quienes son ellos?
Answer:
I climbed that tree from the garden.
I climbed these trees from the garden.
You were flying the child's kite.
You flew the child's kites.
Explanation:
The last two have the determinative article "the", it does not correspond any demonstrative adjective.
Answer:
Antonio Lava su carro los fines de semana.