Leucs are a social frog and a 40 breeder is plenty of room for a group of 5 or 6. If you keep just a pair with a few froglets it should be fine IMO. Lots of leaf litter, caves/ hiding spots will be critical though with lots of spring and isopods. Just watch the behavior and if you see aggression pull the froglets out asap. Would hurt to have a grow out tank as a plan B.
If it were me, I'd avoid it. Not to say it's right or wrong. It just wouldn't be worth it to me.
1) My Leucs are pigs. Heck, they hop all over each other constantly, leap-frogging for FF. While neither intimidates the other, I can't imagine that a froglet is going to get in there without freaking out the first time one of the adults jumps on his/her head.
2) I want froglets in my care to learn to eat aggressively so they grow up healthy. I don't want any distractions. Nothing in the tank that causes them pause or to hang back when food is available.
I have a group of five Auratus froglets (JV's now) and in spite of being in a bigger tank, I still feed in three different locations just to avoid too many froglets in one place feeding.
3) Your tank looks great but still looks a little thin in terms of hides/plant growth. I would add some creepers and some mid-height growing plants to provide additional hides and visual barriers. Seems like the more that my tanks grow out, the more I see frogs out regularly and without fail.
You haven't really told us why you want to do this. If you're interested in the family aspect of PDFs, I would do some serious research/homeowrk and study Oophaga/Pumilio......or any species that cares for young.
To be clear, not saying that it can't be done. Just not a big fan of anything that could potentially slow growth or inhibit feeding of froglets.
Love the moss growth in your tank! I'd dump more leaves on the left side where there isn't moss. That will given them a nice hidey area.
I don't know a lot about Leucs. They are group frogs but I'd still be concerned about a small froglet. How small is the froglet in question? One of those tiny culte ones? If so, maybe make up a nice temp container for him until he's grown a bit.