Their arms were actually about the length of human arms. It's just the rest of them was very big but they almost certainly could manipulate stuff with those arms.
My niece had that phase but other girls didn't like her playing dino and got scared. She got sad because the other girls didn't want to play with her anymore, but kept doing it (and having fun scaring other girls). When I asked she told me she liked to be a dinosaur more than to play with the girls lol .
Someone took Papa Doback's advice to heart.