Rear gears not shifting between 6 and 7 I've just replaced the rear derailleur on my kids bike. I've fitted the biked with a Shimano Altus. 
The issue I am seeing is the shifting between gears 6 and 7. When I move from  7 to 6 the chain does not move onto the next cog. When I move from 6 to 5 it moves to the next cog and then continues up with each gear change. So effectively when I'm at 1, the chain is on the second last cog.
When I shift from the gears from 1 to 7 the same behaviour is seen. So effectively when I move from gear 5 to 6, the chain has already moved onto the last cog. So when I move from 6 to 7 the chain just sits there.

What do I need to do to ensure that the chain moves from the smallest to the next cog when I move the gear from 7 to 6 ?
Most probably the cable is too loose so that the derailleur is resting against the high (outer) limit when in the highest (smallest sprocket) gear, and there is slack that the shifter needs to take up before moving the derailleur to the next sprocket.

Shift the derailleur to the smallest sprocket and make sure the shifter is on the highest gear setting, turn the barrel adjusters in to their slackest setting, undo the cable pinch bolt and pull the cable with a pair of pliers to take out slack and do up the pinch bolt again.

If you have trouble getting slack out of the cable, a trick is to change gear to the second smallest sprocket, then without pedaling click the shifter back to the highest gear so the chain stays on the second smallest sprocket, then re-tension the cable as above.

There are plenty of how-to videos for bike adjustment and repair. Park Tool Company does some really good ones. Their rear derailleur one is here.