There was not an agreed time for me to pick them up
My aunt used to do this to my mum and uncles until they all refused to help any more. "I have a few jobs to do, any chance you can look after for a while today?" They'd agree and my aunt would drop the kids off in the morning and roll up to collect them 7/8 hours later and be shocked that her siblings were pissed off at having had to entertain her kids all day with no idea if she'd arrive in 5 minutes or 5 hours (and this was before mobiles so it wasn't easy to get hold of her to find out).
If I was asked to watch any of my nieces or nephews while my sibling did a food shop, bought a couple of last presents and prepped for xmas eve dinner I would not expect it to take 6.5 hours! At 4 and 2 I'd be expecting your kids to be heading to bed around 7pm so would have assumed you'd be collecting them at around 4/4.30 to get them home, fed, bathed and ready for bed by then. If you collected them any later -without explicit prior agreement or warning - I'd be annoyed.
It’s just that my sister had arranged to do something with my mum and auntie and has specifically said that the children cannot come but thinks o would be happy to come along when she goes for months without seeing them
She's arranged an adult event for her mum and auntie and was polite enough to invite her other close female family member to join. Whether you want to attend or not she has done the polite thing by extending the invite.
The fact she doesn't see your children very often is an entirely different issue - not inviting them to the afternoon tea is not a snub as it is not a child friendly affair.
Equally, she has no obligation to spend any time with your children. It would be nice for her to offer to spend time with them but she is their aunt and can choose to spend time with them or not. Perhaps she isn't fond of spending time with your kids because they are too boisterous or she just doesn't connect with them the way she does to her DHs nieces/nephews and she just isn't close. That does not make her a bad person.