Jen, for both I would do an earlier bedtime
With the 4yo, I would put her down around 6.30-7pm and see if that helps with the early wakings. Also what worked with my 3yo was to give her a cue that its time to wake up,etc.. and with her we put a night light on a timer and I set the timer to 6.20am. Even if she wakes earlier than that time (which does happen), she will stay in her bed until the light comes on, and thats when she's allowed to come out of her room and wake me up. But I really think an earlier bedtime would help... especially if you can see that she's still tired and needs more sleep.
With the 18mo, I would do a nap around 11.30am and he might sleep longer than an hour. If he wakes still an hour into the nap, try to resettle him to get him to sleep longer... and if it doesn't work, then you're probably best to put him down for the night around 6pm. How does he fall asleep for his nap and at night? When he wakes an hour into the nap, do you get him straight away or do you give him a chance to resettle?
Layla