see technically you should be putting the oldest child in the least safe position. so that would be the 5 year old.
I can see the dilemma though, because high backed boosters maybe seem to offer less protection in a crash than a big car seat?
but then the child is younger/smaller, so any impact will have a bigger effect.
i had this exact same discussion with a group of people on FB the other day and it's a really hard decison to make. points that were made during that chat included:
is it safer to have the eldest in the front?
statistically you're more likely to have a head-on collision. But the front seats on modern cars tend to be very well designed in terms of keeping the occupants safe. you also are much further back from the impact, especially if you are able to push the passenger seat as far back as it will go (this will depend on the car seat behind it)
the likelihood of a high-impact rear-end collision is low. But if you were unfortunate to have such an accident then your child would be in the crumple zone. BUT, the Galaxy is a relatively "high" car, so unless the vehicle hitting you was also "high" then the impact may well be quite low down, so less effect?
How easy is it to get children out of the back row if you have child seats all across the middle? If you're hit from behind how will emergency services access the child in the back? time could potentially be wasted having to remove a seat from the middle row in order to lift the chair and get to the back
In some countries (France, I believe) it's against the law to have a child under a certain age in the front seat. But in others it's considered as safe as in the back seats- Scandinavian countries take this viewpoint and their general attitude towards car safety seems to be a lot higher than ours!
Ultimately there is no real answer I don't think! It's all different sets of risks and you just need to weigh them up and figure out what is best for your own family.
For example, I know if I had my 3 year old in the very back row it would potentially be very distracting as he'd be shouting for stuff all the time whereas my 7 year old would be quite happy back there reading/playing games.
As for the original question, I still don't know the answer i'm afraid