I guess in terms of both of them having interrupted sleep, how are they in the day time are there stresses & any changes that can be tied into it happening.
Also how does the 5yo fall asleep ie is the house still light etc, it may be that a night light in a hall way might make a difference.
I've never really had children who got up to go to the toilet in the middle of the night, so not sure how to tackle that but my understanding is that waking in the middle of the night to go to the toilet is kind of in the same basket as bed wetting in that the child isn't producing enough Vasopressin to stop urine production in the sleep (so they aren't technically "dry at night" & so it might be worth the family looking at why she wakes to go to the toilet & working with that rather than her not being able to go back to sleep, ie does she drink too much late in the day, does she have a big enough bladder to hold urine properly in the day to only go 3-6 times in 24hours (normal amount for a school aged child) because it may be that she goes too frequently & there is an immature bladder issue that is behind the waking at night to wee.
In terms of the night terrors, we found with DS1 there was a link between sugary foods late in the day & his night terrors & looking at diet may help. Are the true terrors in that he is still totally asleep & not even acknowledging people around him, DS1 was a bit older but he'd say "I want my Mummy" even though I'd be holding him & he'd have no memory of it the next day, although he was a bit older when he had them. Most importantly though with the night terrors if the child isn't awake is to NOT wake them as that can cause more of them & more anxiety.