Mrs Yeats said it was hard to know whether the speech was a result of the Sativex or other medications, but the family had recently increased her Sativex dose and her speech had improved.
The medicinal cannabis was also allowing the family to wean her off other medication that could affect her cognitively over a long period of time.
Mrs Yeats said Sativex kept Grace calm, alert and focused.
However, the drug wasn't something she could afford without public help, she said. It costs more than $1000 for one month's supply of the drug, which is not government funded.
One Dollar Warriors -- a group which raises funds to fight life-threatening illnesses -- set up a Givealittle fund for the Yeats family and collected enough to pay for Grace's Sativex for two years.
"It gives me two years to fight for other options I guess, because it's definitely something that I can't take away from her now," said Mrs Yeats.
She thought a lot of other people would like to try Sativex for children with spasms but the cost would put them off.
"What's the point of trying something that you know you can't sustain? It's just, it's cruel."
Mrs Yeats said she knew of a woman with multiple sclerosis who couldn't afford the drug.
"She has trialled Sativex and it's been wonderful for her but she can't afford the next lot so she knows that she's going to have to go back to, you know, being miserable."
Mrs Yeats said government funding for the drug would be "a dream come true" for many people. Nationwide, the Ministry of Health has received 76 applications for pharmaceutical grade cannabis products with consent for distribution in New Zealand over the past three years.
Sativex is approved for treating symptoms of multiple sclerosis in New Zealand. Any other use of Sativex is an 'unapproved' use, according to Medsafe.
The ministry approved 73 of the applications including 16 applications for renewal of previously granted approvals.
It received three applications for non-pharmaceutical grade products and authorised two.
One was for an application to use Aceso Calm spray to treat a severe case of Tourette's Syndrome approved this week.
According to the ministry, medicinal cannabis includes several types of products containing extracts of the plant that may be used to treat various conditions.
Ministerial approval was required before they could be prescribed, supplied or administered.
Three types of medicinal cannabis products could be considered for approval.