Wilmington Fire Chief Willie J. Patrick, Jr. issued that ruling on Tuesday morning.
He also said there were no smoke alarms in the home.
The children, a three-year-old girl and a one-year-old boy, were injured in the fire on Friday.
The Wilmington Fire Department said on Monday that the girl was listed in serious condition at A.I. DuPont Children's Hospital and is showing 'much improvement.'
The boy is at Children's Hospital in Philadelphia and is listed as critical but is showing some improvement, firefighters said.
The fire broke out in the second floor of their grandmother's row home in the 1100 block of Pleasant Street around 12:20 p.m. Friday.
Firefighters were on the scene within minutes, and began efforts to rescue two children who were trapped inside.
Only the Action Cam was on the scene as Second Ladder Company Lieutenant Eric Haley and Senior Firefighter Cornelius Skinner first rescued the children.
Haley could be heard yelling "I got one" at the fire scene.
"When he kneeled down on the floor, that's when he said to me, I got one," said Skinner. "I was on my hands and knees when I found the second victim "
"At that point it's almost like your child. You're grabbing that child an you're running full steam to try to get them outside to the help that they need," said Haley. "Once we picked up the children we were holding them as tight as we could and just trying to come out."
The fire was discovered in a second floor rear bedroom and extinguished within 10 minutes. Fire damage was contained to the one house, but the homes on either side had smoke damage.
The toddlers had been in the care of their grandmother, while their mother was at work. The grandmother escaped the burning home without injury.
Since the family lost everything in the fire, a Facebook page has been set up to organize donations for the victims.
For more information, visit the Pleasant Street Fire Survivors Page on Facebook.
In addition, Bliss Elements Salon and Spa at 2400 Kirkwood Highway in Wilmington said it will donate proceeds from a recent consignment sale to help the family. The Spa is also holding a toy and clothing drive starting Tuesday.
The following is a list of the ages and sizes of the family:
1 year old William wears 18-24 month clothing and size 6 shoes
3 year-old Saniyah wears 3T-4T and size 9 shoes;
7-year old Nambia (girl) wears 10 ½ clothes and size 3 shoes;
9 year-old Camden (girl) wears 10 ½ clothes and size 3 shoes;
Their mother wears size 10 pants and size 9 ½ shoes;
Their grandmother wears size large clothing and 8 ½ shoes.
Bliss Elements Salon and Spa said it will collect items during regular business hours (Tuesday - Friday from 10:00 a.m. - 6:00 p.m. and Saturday from 7:00 a.m. - Noon).
For more information, please contact Sonji Hubbard at 302.543.5469.