We went to Angkor Wat last September from Bangkok. I don't know what immigration issues you speak of. You can easily get a visa ahead of time online or on arrival. Also, 5 days is way too long in my opinion. Especially with kids. Other than the temples, there is very little to do in Siem Reap. And you can only see so many temples before they all blend together. And I imagine with kids it will be even more difficult to keep their attention. It's an amazing place and definitely worth seeing. But with kids I'd go for two days max. I personally went for one day and was templed out by the end of the day. But that's just me.