Hurtful comments to childless people

It is a very long time since I had babies and small children in the house, but I do remember very clearly that having to be awake and dealing with them night after night, after a long day, was the most tiring thing that I have ever done, before or since. That includes looking after a terminally ill husband at nights (though sadly that didn't last for the years that an insomniac toddler could keep going)

I would second that ^"you don't know what tired is until you have kids" - the other colleague may well think that ^"you don't know what tired is until you have an elderly sick mother", but BOTH situations are exhausting.

That is not to say that other people don't face other tiring things when they have elderly family members to look after, but when you have had several years of sleepless nights with successive toddlers who can't even be reasoned with, that is at the forefront of your mind when someone mentions being tired. You are very much aware of how you are affected!

I am sure this young mother wasn't deliberately being hurtful. She is working by day and trying to catch sleep every night while an energetic toddler turns night into day. Cut her some slack!

I did remark casually when we were on our own that I felt sorry for older colleague, it must be very stressful having to look after an older and sick parent and she is looking very tired.

Hopefully it might make her think.

I don't think the mother had meant to be nasty in any way, more a slip in judgement as we get into saying these things without forethought!
Could you possibly have a little chat with her, reminding her that not everyone has had the chance to become a parent and may regret this for the rest of their lives!
Don't be nasty to her, but ask her that she may see it from another point of view in future!