Seven people, including the five children, were taken to hospital after a corrosive chemical was mistakenly used on a slip and slide at Kindercare Woolston on Friday afternoon. Two others also received minor injuries.
In a statement, Kindercare said the centre reopened this morning.
"We are overwhelmed at the trust families have placed in us, despite Friday's incident, and we are encouraged to see that all of the children who attended hospital on Friday, have returned to our care today. We will continue supporting our families and team.
"Our investigation is underway and we're committed to working cooperatively with WorkSafe and the Ministry of Education through this process."
Kindercare said it would not be making further public comment until the investigation had concluded.
In a letter to parents, Kindercare admitted it had failed on safety.
The provider's chief executive and regional manager flew to Christchurch on Friday night.
Kindercare said the chemical involved was used in many of its centres and was normally kept in a kitchen cupboard or laundry and plumbed directly into the centre's steriliser.
The company said it wanted answers on how the chemical was taken from the storage area and used in a play activity.
WorkSafe was also investigating.
On Friday, Fire and Emergency senior station officer John Herriot said the burns were as a result of a mix-up.
"The teachers have had a waterslide - bit of fun for the kids - out the back today and at some point of time some detergent's unfortunately been misidentified and a corrosive product has ended up on the slip and slide. This has caused some irritation to the children's skin and some light blistering," he said.
St John assessed 40 patients in total.
"Eighteen units responded, nine ambulances, three rapid response units, five operations managers and one Major Incident Support Team vehicle," St John said.
St John national operations manager Chris Harrison said the patients had suffered chemical burns and blisters.
"They were on a kids' playground slide, which the product had been poured down, and then they've slid down the slide and then the teacher that was injured was at the base of the slide in a paddling pool," he said.
