Hampi can be hot, dry and dusty throughout the year. For this family day trip, pack sunscreen, scarves, hats and plenty of liquids. Wear light, full-sleeved, full-legged cotton clothing and comfortable shoes.
It is easy to get lost in the crowds, so keep your kids close. The crowds can get overwhelming if you aren't used to India. Everyone we met was friendly, but had different definitions of personal space. Keep your eyes, ears and mind open and you'll have a good time.
November through February are the coolest months (by coolest, I mean the least hot!) to visit Hampi. Try to avoid the last week of December and the first week of January. These weeks can be especially crowded, given that schools are out for Christmas and New Year.
Book a local tour guide and a taxi (or cab with a driver) for your family trip to Hampi in advance. Negotiate for a good price. If you wait until the last minute, you will be at the mercy of aggressive salesmen.