Actually, Mozart did write the music that is known as his first symphony (K.V. 17) himself. It is not a very complex work, but all sources indicate that the young Mozart (he was eight years old at the time) wrote the thing over the course of a couple of days while touring England with his father and sister.
Mozart's second symphony was definitely composed by his father, but it is hard to hear many (if any) stylistic differences, because the format of the music is the old classical style. Wolfgang Mozart had as yet to develop his own (rather unique) style.
The music known as the third symphony (K.V. 19) was also not written by Mozart. It was wrongly ascribed to him, but the symphony was actually written by another composer of the period, C. F. Abel. Again, it is relatively easy to see why it is difficult to decide who wrote what, because of the classical style involved. However, it has to be said that Mozart was able to achieve the level and skill of composition of an adult when he was actually still a young child.