I wouldn’t say it’s neglect in the legal sense. It’s only for a few hours. And it appears the baby was generally well cared for. And they ‘forgot’ to give a drink, rather than didn’t realise it needed one or deliberately not give it. Context is missing but maybe a mix up and one person thought the other had given milk.
But the main person left to care for the baby has been neglectful of its needs by not offering a drink. The baby was offered food but refused to eat all of it. So, someone DID feed the baby. Just not the specific food the mum left.
If the carer was the dad and PILs, he needs to look after his own child more, so he understands its needs and learns to be responsible. This shouldn’t be a reason to never leave the baby with him again (which he may be quite happy with). Unless there’s serious danger, neglectful parents need support to learn how to take responsibility for meeting their baby’s needs.
But TBF there could be important context missing, such as (menopausal - forgetful) MIL took over and said ‘leave it to me’, then forgot the drink and offered her own food the baby refused, then made an egg. But if the dad was there, it’s his responsibility to look after his own child. He shouldn’t be leaving to others without checking.
Also depends on context of how the carers feel. If they feel terrible and apologetic, it’s disappointing, but not something to have a huge family fall out over. If they’re doubling down and brushing off the lack of drink and not using the food left by the mum, a serious chat needs to be had before they look after the baby again.