There are reasonable answers to all the questions that are being asked.
- Why didn't the other children notice?
Maybe they were asleep, or very young, or sitting on a different row, or absorbed in what they were doing (if being bribed with the ipad DS wouldn't notice if we had all been abducted by aliens), or they have SN that meant they didn't notice/couldn't communicate.
-Why didn't they ask her if she wanted a drink/wee or check her for nose bleeds and vomit?
She is three and eloquent enough to say 'I saw Daddy's car driving away' so she can presumably say 'I'm hot', 'I need a wee', 'I need a drink', 'My nose is bleeding' and 'I've been sick' so probably doesn't need to be constantly asked. I assume they took her silence to mean a. she was ok or b. she was asleep.
-Why didn't they look back and check?
Maybe they couldn't see her if she was in the back row and/or in a rear facing seat. Maybe all but the driver were asleep.
-Why didn't they strap her in when they set off?
Well they both thought the other did it obviously. Or one of them did do it and she got out again and they didn't realise (We have 4 and this happens a lot. You get two in and then go and look for the missing two, come back with them and one of the others has got out to get something, go back for them and one of the others gets out!).
-Why didn't they stop for the next two hours?
Because they were trying to get somewhere! No-one stops every two hours on a long journey.
I can see how it could happen. I don't think there is any reason to suggest they did it deliberately.