Lee-Anna Shiers, 20 and her son Charlie, 15 months, died in the blaze in their top floor flat in Prestatyn. Also killed were her nephew Bailey, four, and niece Skye, two, who were staying on a sleepover treat. Lee-Anna's partner Liam Timbrell, 23, was rescued, and told a paramedic Smith was responsible, before he later died in hospital.
Ms Smith, 43, was accused at Mold Crown Court of starting the fire last October in a row over a pushchair left in a shared hallway. She will be sentenced in May.
The court was told that Smith has argued with Ms Shiers because the mother-of-one left her son's pushchair in the downstairs communal hallway at the property. Smith had previously made threats that she would set Ms Shiers' house on fire "with you and your kids in it".
On the day of the fire Smith had been drinking and set fire to the pushchair using a lighter. Ms Shiers and her family were trapped in their upstairs flat.
The jury heard harrowing evidence of the scene from neighbours and Mr Timbrell's 999 call was played.
Speaking outside court, Ms Shiers' mother Joy, 45, said: "Anna was such a lovely, bubbly girl and Liam a very caring dad. Both of them were heroes because we know they would have done everything to rescue the children.
"The three kiddies loved each other, they played together all the time. They were our angels. Really, really nice people that will be missed."
Mr Shiers, 64, added: "Nothing will bring them back but justice has been served."It's going to be very hard to fill the space that has been left in our hearts and in our lives."