Assuming that she doesn't have hyperemesis or is carrying multiples pregnancy is not a good reason not to do her job properly / to neglect your daughter. She is 4 months away from her due date. She will have to figure this out soon enough with her own child that feeling a bit grotty still means that you have to get on with it.
I don't think you should be offering to fund another nanny while she has paid time off unless she is taking it as her holiday entitlement simply because you are uncomfortable having a difficult conversation.
It is really much easier to have a difficult conversation when you essentially write yourself a crib sheet to work to.
I think you need to have a gentle but formal conversation about this and soon. Something along the lines of...
"Hi [Nanny], I am going to come home a bit early today so that we can have a chat this evening after work before the weekend. I should be home about 5.30 and DD can have some quality time in front of cbeebies. Can you please make sure that she's finished eating by then?"
"I wanted to talk to you before the weekend about a few issues so you have time to think about them overnight/over the weekend and we can talk again.
The first thing is quite a big issue and that's to do with your timekeeping. I appreciate that you are feeling grotty and I know how exhausting pregnancy can be but the current situation is not sustainable. I have altered my hours to try to make life easier for you but it has not improved matters. Due to persistent lateness I now have had a verbal warning at work. Any recurrence within 6 months and I will be given a formal written warning before dismissal. [laying it on a bit thick but to be honest, I think you are being taken for a mug now]
I know that children go through phases but I have noticed that Dd's behaviour is becoming quite poor and her diet is not what it should be. I would like to see you ensuring that her meals return to a more varied diet and that you pull her up on her behaviour as you always used to do. She will find it even more difficult when you leave if your maternity cover has to be an evil disciplinarian. Please stop giving her sweets except on X occasions and now that she is getting older, I think we should be encouraging her to tidy up after herself.
I need you to understand that the situation needs to really improve or I will have no choice but to give you a formal written warning. [this assumes you have a written contract in place]
Can you please have a think and let me know what you want to do. You will need to find the energy reserves to improve things as I can't afford to lose my job. Is there anything I should be aware of as your employer? Is the baby ok healthwise, is anything worrying you? "
Then, depending on her response you can suggest that she take a day or two off a week as holiday or goes part time. If she jumps at the idea of going part-time then tell her that you will look into it and will approach her maternity cover to see if she has any spare days at the moment. You are sticky ground if you appear to be pushing her out the door just because her maternity cover is available. She needs to be given the opportunity to step up or face dismissal in my view. That said, you have a duty of care as her employer but it sounds as though you have gone out of your way to date to look after her.
Have you considered whether she can return as a NWOC? Nanny with own child? If you have rejected it as an option, she may have mentally switched off. If you have not rejected it or not discussed it, then perhaps now is the opportunity to do so - ie there is a bloody good reason to improve because her job is waiting for her without a requirement to find her own childcare.