Agree, but Tylee was on the cusp of a new life away from them as an adult, and J.J. could have lived and thrived with the Woodcocks, so their baggage could have been safely stowed elsewhere, while they lived in paradise with all or a decent chunk of the insurance.
Either they wanted it all, or they were invested in a cleaner break than that. This, IMO, was where their culty thinking dangerously intersected with other options that involved TR and J.J. living but still a threat, as zombies or witnesses or whatever. I don’t get it but it must have made sense to them, enough to kill kids for.