I cannot add anything else to the thread for advice, as many posters have given you great advice ... Put it IN WRITING to the woman (email and hard copy and keep one for yourself, and maybe copy your manager in) saying you are NOT changing your working hours. And if SHE needs to change/drop her hours, she needs to discuss it with the manager - and add that you want her to stop haranguing you to go full time so SHE may drop her hours. Otherwise you will take the matter further and report her for bullying.
I had a similar problem some years back. I was full time for 6-7 years, then I had my daughter and went part time (2 days one week/3 days the next... so 2.5 days a week.... working for the council.)
And my issue wasn't with colleagues, but my manager when my daughter was a YEAR old. I had been back off maternity leave for about 7-8 months as you only got 18 weeks back then... And at this point (when she was a year old,) he told me that now my daughter is one, I can go back to full time, because there is no need for me to be part time anymore! 
I was aghast and said 'what? She is only a baby still.' He said 'your childminder has her 2 days one week, 3 days the next, she can have her full time. You need to come back full time.' I don't know why, because someone else had been taken on full time, to take on my extra work when I went part time 3 months before I went on maternity leave, and to take the extra work for another woman who dropped from 4, to 2 days a week. Everyone was happy, yet he kept nagging me.
My contract had been CHANGED to 2.5 days a week, (by the manager above HIM 3 months before I went on maternity leave,) and there was nothing he could do about it. So I said no, and after that, he kept on bullying and harassing and haranguing and nagging for me to go full time. 'She doesn't need you at home all the time, you NEED to come back full time blah blah blah. ' He wouldn't let up.
After a YEAR of tolerating his bullying, I reported him to HR and he got a warning for it. He tried to make my life hell after that by attacking everything I did, so I reported that too, and he got told off again, but he still never stopped having a go at me.
I move departments shortly after, as I couldn't stand it any longer. But I remember the bullying well, to go back full time.
If this woman doesn't quit when you tell her in writing to cease and desist, then take it further.