Which type to use for a flexible array?

[AlectronikHQ]

I need to write a tree structure in nim. To hold the children of a node, in C I 
would use an array and an int, specifying the number of currently allocated 
slices. Which datatype to use in nim? Nim arrays are compile time sized and the 
tree should support from no/one children to thousands, so compile time size is 
no option.