Shared and private vans leave Aswan around 4–5am; the desert road is closed to tourist traffic between 5pm and 5am, so plan to leave Abu Simbel by about 4pm.
By air
EgyptAir and Air Cairo fly from Aswan in about 45 minutes, with a shuttle to the temples. More expensive, much less tiring with children.
Sun festival
On 22 February and 22 October sunlight reaches the inner sanctuary. Not in January, but useful if your dates are flexible.