Could this be Rusty's Space Ship (1957) by Evelyn Sibley Lampman

There was no way for Rusty Adams to know that the shiny metal disc he had found at the city dump and used on his play space ship in the garage, was actually the flying saucer of the Mighty Gwump of Eopee in Adromeda Galaxy. When tiny Tiphia, Gwump's messenger, arrived to claim it, Rusty and his playmate Susan Northrup found themselves taking an unexpected trip to all of the planets, except Neptune and Plato. Rusty and Susan save Tiphia from attack by ants and other hazards as he vainly tries to find the moon, Eopee. It's Rusty who realizes finally that Tiphia is simply in the wrong galaxy. This is a Baedeker of Outer Space.

Could this be Rusty's Space Ship (1957) by Evelyn Sibley Lampman?

There was no way for Rusty Adams to know that the shiny metal disc he had found at the city dump and used on his play space ship in the garage, was actually the flying saucer of the Mighty Gwump of Eopee in Adromeda Galaxy. When tiny Tiphia, Gwump's messenger, arrived to claim it, Rusty and his playmate Susan Northrup found themselves taking an unexpected trip to all of the planets, except Neptune and Plato. Rusty and Susan save Tiphia from attack by ants and other hazards as he vainly tries to find the moon, Eopee. It's Rusty who realizes finally that Tiphia is simply in the wrong galaxy. This is a Baedeker of Outer Space.

Could this be Rusty's Space Ship by Evelyn Sibley Lampman

There was no way for Rusty Adams to know that the shiny metal disc he had found at the city dump and used on his play space ship in the garage, was actually the flying saucer of the Mighty Gwump of Eopee in Adromeda Galaxy. When tiny Tiphia, Gwump's messenger, arrived to claim it, Rusty and his playmate Susan Northrup found themselves taking an unexpected trip to all of the planets, except Neptune and Plato. Rusty and Susan save Tiphia from attack by ants and other hazards as he vainly tries to find the moon, Eopee. It's Rusty who realizes finally that Tiphia is simply in the wrong galaxy. This is a Baedeker of Outer Space.

Could this be Rusty's Space Ship (1957) by Evelyn Sibley Lampman

There was no way for Rusty Adams to know that the shiny metal disc he had found at the city dump and used on his play space ship in the garage, was actually the flying saucer of the Mighty Gwump of Eopee in Adromeda Galaxy. When tiny Tiphia, Gwump's messenger, arrived to claim it, Rusty and his playmate Susan Northrup found themselves taking an unexpected trip to all of the planets, except Neptune and Plato. Rusty and Susan save Tiphia from attack by ants and other hazards as he vainly tries to find the moon, Eopee. It's Rusty who realizes finally that Tiphia is simply in the wrong galaxy. This is a Baedeker of Outer Space.