How can I ensure my marker tips don't get destroyed?
I have seen children grasp markers in a clenched fist and vigorously fill in large swaths of the drawing area with colour. It's the vigour with which the children fill the area that destroys the marker's tip. 

If this is what you are seeing then there are a couple of things you can try to help the children learn focus and finesse when making art ... and protect the markers.


Break up large areas into smaller ones

Never have an image which requires to be filled in all with the same colour. Say for example, an all blue table cloth is instead made to have a checker pattern. The smaller checker pattern forces the artist to focus on a smaller area. The strokes become more controlled and the child's hand movement is slowed. You see this being done in the adult colouring books where the image is intricate requiring the grown up to slow down, focus and concentrate in order to stay within the lines.
Teach them to hold the marker like a brush and not a pen ... or ice pick

Teaching the children to hold the marker as a painter would hold a brush will teach them a gentler way of applying colour to a surface. By changing the way the marker is held will reduce the amount of force being applied to the tip. 

Another thing you could try is to have the children draw at easels instead of tables. Harder for the child to bear down overtop of the marker and drive it into the drawing surface.
Less abrasive paper

A heavy tooth paper should be avoided as it will abrade the markers foam tip over time. Try finding a paper with a slicker surface. One that will allow the marker to glide freely.