Hi Rach, If they are seedlings they simply may not be old enough. I know P. lusitanica is popular but would you consider pulling them out? They can be quite an invasive weed once they start producing cherries and the foliage is lethal to livestock. They grow happily in native forest and blend right in, with the shade leaves being similar to the herb 'bay', Laurus nobilis http://www.nzpcn.org.nz/flora_details.aspx?ID=3381. P. lusitanica leaves have high levels of cyanide and are quite palatable to livestock, I know one farmer who lost his prize bull to this plant and I had a standing row with a DOC weed ranger who was adamant it was native (as understorey to kahikatea forest), which shows how well it blends in.
A good alternative for the Hutt would be pigeon wood, Hedycarya arborea, http://www.nzpcn.org.nz/flora_details.aspx?ID=805. It might be a little frost sensitive when young but will stand clipping, attractive flowers and fruit too.