She said the dog not only gives Melissa a companion, but also provides an "extra pair of eyes and ears" to keep her safe and let her have a normal childhood.
"It gives her something to be proud of ... The dog kind of takes away the bad aspects of having a chronic condition," Mrs van Blerk said.
The dog has been trained to detect when the 5-year-old's blood sugar levels are falling dangerously low.
Mrs van Blerk said Cleo provided a safety net to allow her daughter more freedom, despite her condition which has no cure.
Melissa, a Year 1 pupil at Dairy Flat School, is also thrilled to be the "poster girl" for this year's Assistance Dogs New Zealand Trust's annual appeal.
The week-long appeal, to raise funds to provide assistance dogs to children with disabilities, will begin this Sunday at 1pm with a charity dog walk at One Tree Hill Domain.
Warriors and Kiwis captain Simon Mannering, who will help lead the walk, said that as a dad, he appreciated the work the trust does.
"I've been learning about how having an assistance dog can give these kids back their freedom, and allow their whole family to enjoy everyday family activities which were previously impossible for them" he said.
The charity walk, called Woof&Walk4ADNZ, follows a short course that will take about 20-30 minutes. Entry is $10 per dog.
Wendy Isaacs, national funding manager, said the life cost of an assistance dog was $48,000.
There was a waiting list of 20 people, and the waiting time was two years, she said.
"There is a huge demand for assistance dogs ... these dogs are trained to enrich the lives of adults and children with disabilities as diverse as autism, diabetes or cerebral palsy," Ms Isaacs said.
So far, the trust has released 25 dogs to families in need, but fields eight to 10 new enquiries each week.
"Without money generously donated by the community, we can't carry out this very important work," she said.
The Assistance Dogs New Zealand Trust is looking for volunteers to collect in Auckland on August 7 and 8 for the national appeal week. If you can help, please contact Wendy Isaacs by email at wendy@assistancedogstrust.org.nz, or phone 022 137 4090.