So cute!

Good idea to keep an eye on them, especially the one that may not be nursing as much as the others - if s/he starts to look wrinkly with loose skin, that's a sign that s/eh isn't getting enough milk. If s/he needs supplemental feeding, what I do first is try to get the doe to feed a little extra - just place the kit under her and watch very carefully to make sure the doe doesn't injure the kit trying to get away. She may allow the kit to attach and feed for a bit (and every tiny bit helps since rabbit milk is incredibly nutritious).
If that doesn't work, you can try hand feeding with kitten milk replacer...but I hope that isn't necessary and that all the little ones grow like weeds.