Tell your daughter you are willing to buy any car seat she may wish to recommend so that you could take the toddlers out occasionally to give her a break and "its completely up to you if that is something you would like to help you out" Ask her what might help- she may or Amy not have suggestions- she will either open up a discussion or close it down. Ie open, none judgemental equations, not implying any criticism.
The decision is your daughter's whether to accept of reject any offers of help and support.
If not just let your daughter carry on complaining she is struggling- she may just want to sound off and complain to get things off her chest.
If the father is on the scene maybe have a quiet word with him- sharing what your daughter is saying and discussing the situation- again in a none judgemental, none critical way- just how are things? etc
If he's not open to that discussion- accept the status quo. On visits perhaps you could take some cooked food, e,g like a lasagne, shepherds pie etc that could be a family meal the next day. That saves cooking one meal at least! I would have been overjoyed if my mum had done that after chid birth- instead she brought cream cakes and she knew I didn't like cream! 😊