Cops say the 37-year-old was supposed looking after Alyssa Lynch, eight, and her five-year-old brother Caleb while their mom and the woman he was supposed to marry, Lisa was at work.
She told them they'd planned for him to drop the kids off at her parents house, before he went on to his job.
But the youngsters never made it - both of them passing away in the ferocious blaze that raged through their Evanville, IN home.
The flames were so intense that it took cadaver dogs several hours to locate the children's charred bodies, the Courier Press reports.
One of the children was found in the bedroom with burned flares near or under the body according to the legal affidavit.
At the time officials also revealed there, "may be evidence the person had been bound in some fashion with duct tape."
When they caught up with Weisheit, he led them on a high speed chase, during which he tried to kill two officers, cops alleged.
When they finally brought him to a halt, he threatened them with a knife, but they brought him down with a stun gun.
A not guilty plea was entered for him, by his assigned public defender.
But if convicted Vanderburgh County Prosecutor Stan Levco said: "Obviously this is death penalty case. It's some that I'll be considering in the next two to three weeks."