Whether it's true or fabricated, I think the parents have presented a plausible scenario.
If the last day of school before winter break was 12/18, 12/19 would be a good time for a visit to grandma's - a fun change of pace for the 4 older kids. (Keeping the younger two home makes absolute sense to me for many reasons, several of which have already been discussed.) Making a shopping stop en route also seems reasonable with Christmas coming up. Taking 6 kids on an outing isn't unusual when you have 6 kids! The older 4 are dropped off, and the fam returns home.
The babies have less to do without the older kids around. Dad goes to collect some kindling, which takes him in & out of the house, and in & out of the yard. The boys ask to play on the back porch; Mom uses the opportunity to get some wrapping done. Between all the in & out, Dad loses track of time until it occurs to him he doesn't see/hear the boys. His first instinct/hope is that they're close by, so he checks the yard & house before doing a quick search around the block - feeling the confusion/denial/fear many of us have felt when we can't lay eyes on our child but still have to believe they are safe and nearby. Not finding them in the immediate vicinity, he returns home & calls 911.
I think it all COULD be true, and, for me, nothing so far (tinted windows, face masks, crossed arms, lack of public appearances, the word "rambunctious", not releasing photos to the media, staying quiet, etc. etc.) rises to the level of evidence that the parents are abusive, violent or guilty. That evidence may well come out, but for now, it's the single fact that the boys are gone that raises suspicion.
It's possible the boys never made it to CalCity, but that's a long time to hide their absence, and a long time for the other kids to keep the secret (though admittedly easier in the days of COVID). I lean toward thinking that an incident occurred after the older kids left, possibly the night of 12/19. I think both boys likely suffered the same fate, which leads me more toward an Andrea Yates scenario than, say, a Patsy Ramsey, and pondering the lengths a husband might go to protect a wife.
Guilty or innocent, I think staying out of the public eye is a wise choice for them.
JMO