Sleep sacks and warm clothing!
http://www.sleepsackstore.com/ example I'm not recommending them as a source. In our case all the sleep sacks we had were given to us or bought second hand given the kids out grow them pretty fast but they keep them warm!!!
We didn't do shoulder season trips till the kids were old enough to communicate with us. Primary reason that they can tell us if they are cold etc. Oddly enough it was our Red Doberman who nearly froze one night and woke me up standing over me shaking and nearly froze. Spooning with your 75lb Doberman half the night to get him warmed up makes for an interesting camp story. Took him 45 minutes to stop shivering. Wife never woke up she sleeps through anything and everything. Next morning I woke up to my wife asking if there was something she should know about regarding me and the dog. HA!
Poor dude was cold! Had him 16yrs he was an awesome dog! I have our Red Dobie #2 crashed out behind me he'll be 2 in June. Hasn't been camping yet should be interesting this summer.