Assuming the parents aren't lying about the 11 year old and they had no reason to think she would leave the baby, and they hadn't brought her along and expected her to do it for the whole day, I think this could be a case of a reasonable decision going wrong and they shouldn't be criticized for it, they can learn from it as most parents do when they realise their judgement was wrong. And that's enough.
I would expect most 11 year olds to be capable fo staying with a baby for an hour or so. It doesn't sound like a great day out for the 11 year old (though who knows, maybe that was part of a negotiation - "We can go to Thorpe park, but you will need to stay with the baby for a third of the time, we'll do things as a family for a third of the time and we'll do what you want for a third of the time.). A theme park isn't a terrible place to give an 11 year old a bit of responsibility, it's a family friendly environment with lots of staff around. 1 - 1.5 hours is a fairly long time though and on the edge of what I would think was acceptable in normal circumstances.
Of course if they made the 11 year old up, or didn't actually tell her to stay with the baby, or knew she was flaky, then that's another matter. Or they'd come into the park and left her with the baby for 3 hours already and weren't intending coming back for a lot longer, or if they constantly use her to babysit while they have fun and don't give her the attention and care she deserves, then that's another way this could be really poor parenting. Parenting decisions can be quite complex and you don't know from this report whether it's a culture of expecting maturity from the older daughter or a culture of selfishness.