I ordered the Kidzlabs Velociraptor for my 7 year old daughter. She's mad on archaeology and this was a darn site cheaper than a metal detector! It kept her happy for about 4 hours solid, as she sat there until she had excavated it completely. If you had a child who only concentrates on things for a small amount of time, it could easily last for weeks.
I would highly recommend either doing this outside, or at the very least in a hooverable room as the dust it creates gets absolutely everywhere, and according to the packaging, can stain. But the excavating itself was easy enough for her to do, but just hard enough to keep her interested. She was able to put the parts together herself afterwards and it made a nice little model for her to keep in her room. ~Well worth the money.