How can I get rid of tree roots coming under my fence from a neighbours tree as i want to lay a kids sandpit?

We have just freecyled flags that lay on our side of the fence only to discover that there are exposed tree roots. We have bought a PLUM Octagonal sandit to lay but are "stumped" as what do do next? Do we need to buy more flags, can we just lay garden weed- preventor material- do we need a tree surgeon? Please can anyone help with our queery?