There are some physical advantages to having your kids young - less likely to be complications with pregnancy or birth, less likely to have birth defects, more energy coping with baby/small child. And I'm sure we all know women who put career first with the intention of starting a family later and ended up finding it was too late to have a baby at all, or who only did so after going through the expense and emotional turmoil of IVF.
Another silver lining for your family: the future GGparents will get to see at least one baby from the next generation.
Is D definitely planning to be a SAHM, or is she going to put baby in daycare and continue her career?
You don't have to have studied economcs at all before, but a poor score (the 3) is much worse than not having taken the subject at all. I agree Calc BC would be very helpful.
Are you genuinely equally interested in all three subjects? There are increasing numbers of other unoversities in the UK offering PE or similar combinations, which are less challenging for admission than Oxford.