Catherine, Duchess of Cambridge and Prince William, Duke of Cambridge take part a tree planting as they visit the National Arboretum.

Catherine, Duchess of Cambridge and Prince William, Duke of Cambridge take part a tree planting as they visit the National Arboretum. Photo: Mark Nolan

Those cute little twin boys who helped water in the oak planted by the Duke and Duchess of Cambridge in Canberra on Thursday morning came from a local family with a power of strength and love behind them.

There are five children in the family including the now-famous six-year-old twins Oliver and Sebastian and all are battling with the reality their mother Moira has terminal breast cancer.

She said days such as the one at the National Arboretum Canberra, in which their twins were centre stage with the royal couple, were cherished, just like any other day.

Catherine, Duchess of Cambridge and Prince William, Duke of Cambridge watch two children help them with a tree planting.

Catherine, Duchess of Cambridge and Prince William, Duke of Cambridge watch two children help them with a tree planting. Photo: Mark Nolan

She is a former employee of ACT Health and helped to establish the Canberra Hospital Foundation to raise money for the hospital.

‘‘I’ve been fighting breast cancer for 3 years and last week we stopped chemo,’’ Mrs Lye, 41, of Holder said.

‘‘I know it’s going to take my life. I’ll just keep fighting with whatever treatment they give me. I’ve found I’ve felt very sick over the last three months.

Royal encounter: Catherine, Duchess of Cambridge, greets twin brothers Oliver and Sebastian Lye, at the National Arboretum in Canberra.

Royal encounter: Catherine, Duchess of Cambridge, greets twin brothers Oliver and Sebastian Lye, at the National Arboretum in Canberra. Photo: Alex Ellinghausen

‘‘I’ve got five kids to keep me going so that’s exactly what I do.’’

Sebastian was also born with Down syndrome and has sight and hearing difficulties. Oliver put a brotherly arm around him as they watched Kate and William toss dirt on the English oak tree.

They both used matching red watering cans to water in the tree, Sebastian exuberantly calling, ‘‘Hi Mum!’’ as he did so.

William praised the boys for doing a good job - "well done guys" - and said the tree would  be "very healthy " after the thorough watering.

Mrs Lye, who attended the event with husband Greg, and their other children Madeleine, 15; Zachary, 13; and Harrison, 10; said all the children doted on Sebastian, who attended the Malkara special school at Garran.

‘‘They are so proud of him,’’ she said.

Oliver and Zachary attend St Jude’s Primary School, Zachary is at St Edmund’s College and Madeleine is at St Clare’s.

Mrs Lye did not meet the royals but said it was enough to see her boys perform to beautifully alongside them.

‘‘I think Sebastian was more excited about being allowed to play in the playground and Oliver thought it was pretty cool. I don’t think they will realise the significance of it until later,’’ she said.

‘‘It was such a wonderful day for all of us, to be able to create memories like that. But every day is special to us, we cherish every moment.’’