I don't think it's been made really clear how long LE has already spent with the children, or whether the children are traumatized right now. They're traumatized to some degree, I'm sure with their sister being missing, but I don't know how much.
The only reason, IMHO, they want to interview the kids right now is to get the kids to change what they already said. I think we can all project what the kids said - Lisa was in her crib, they didn't hear anything, didn't see anything. I think we all pretty much can agree that's what the boys apparently said.
So the goal now would be to get the kids to move OFF that statement, and say they were covering up, and they saw or heard or know something. LE doesn't have fresh questions that they just forgot to ask, and would like the chance to ask something completely different, i.e., your friend's teenage brother has a couple times been rumored to have acted inappropriately with the baby, can you give us information about that?
If they were my kids, I wouldn't want to put them through that. If there WAS something completely different to ask the kids, my guess is, they'd be open to those kind of questions.
At the VERY LEAST, if these were my kids I'd insist on a CASA volunteer being in the room, and IMMEDIATELY stopping any kind of shenanigans meant to fool the kids and trick them into agreeing their mother killed Lisa, or there is evidence of that. A half hour, no-shenanigans interview with each I might approve.
The thing is, they've lost their precious baby girl. They're trying to hold together what family they have left, and LE is quite good at coming in with big wrecking balls and destroying families in order to try to get them to turn on each other.