Hi There,
Sounds like your kids are over-tired. They need a consistant nap/rest schedule. My three and 1/2 year old daughter does not take naps any more, and stopped taking consistant naps well before her 3rd birthday... she will also take naps in the car if we are out in the late afternoon. It is reasonable that your daughter does not nap at this age, but then she needs plenty of rest at night and a decent "rest time". Every day at about 1pm (when my son naps) I read some stories to her and then let her watch a disney movie or sesame street for an hour. If your daughter is not one to watch tv, restrict her to her room with limited quiet toys for an hour or so every day... they need the down time. We implement the car ride nap fairly frequently ie. if we have somewhere to be in the evening, or if she seems really tired. However, we limit how long she sleeps (usually about 1 hour to 90min). Her bedtime with no nap is about 7:30... with a nap 8:00. You have to just be consitant and stick to a routine. Also, if she is getting wired later in the evening, she is overtired at that point. It sounds like 7pm (like you have said) would be a good starting point as a bedtime for her. Maybe nurse your son a little earlier in the evening, then do bedtime routine for both of your kids (ie. bath, quiet play, books) then bed for both of them. This is probably more efficient and better for your son because you are not nursing him to sleep (STOP this habit now... it will only make it harder for him to fall asleep on his own later). Just make a new routine that is efficient for you, and stick to it, your kids will ajust... just be CONSISTANT. Also, both of my kids wen to one nap at 1year old. It is a hard transition because they tend to get tired early, but then they won't take the afternoon nap. The key is to push them a little bit each day. If he seems tired at 9am, push him until 10... ie. go outside, give him a snack (a little sugar boost helped prolong my kids a bit). Then after a few days he will ajust to this nap time, then push him a little longer for a few days etc. until his one nap is midday (I usally put my son in after lunch... about 12:30 or 1:00 (he is 18mos). It is great when you get to the point that your daughter can have rest time while your son is napping! Good Luck and stay strong!