I don’t see the relevance of the maternity aspect. You had enough savings to take the full maternity, what are savings for if not to spend how you like in a way beneficial to you or your family? (As long as all bills are paid etc which you have already stated is the case)
The time with your new baby is irreplaceable and more valuable than any thing (as long as you could afford to do it and pay for the baby/bills which you could)
You also have had enough savings left to pay for an emergency (the boiler) and still have a relatively large amount of savings compared with many.
Unless your job is incredibly unstable, you will hopefully be able to slowly increase your savings again soon. A brand new boiler should last a long time and boilers tend to be the emergency item so it’s unlikely you will need to dip in again for a while.
It is great to have savings and a buffer but it sounds like your upbringing has caused you to have a skewed view of your finances.
If this is the first time you have asked your parents for help then maybe it has caused them to think you are struggling to manage your finances but the reality is your finances are fine and you are being over cautious. Again nothing to do with maternity. Although some people can’t understand why anyone wouldn’t want to be knee deep in spreadsheets while still delivering their placenta…
One option is to say to your parents that you messed up your calculations for tax or some such thing, that you actually have plenty spare and to pay for your own boiler.
Other option, as others have said, is to keep the loan and suck up the lecture. (about finances but not about how you choose to spend time with your baby)
Option 2.5: Take the loan and whatever loan schedule they suggest then pay it back at a higher rate or ahead of schedule each time so it seems like you have made an astonishing turnaround with your financial management. Hopefully allowing them to pat themselves on the backs about how generous they are but how they set you back ‘on the straight and narrow’ with their lecture.