The Saturdays star faced a backlash when she posted a picture on Instagram of five-year-old Aoife - whom she has with husband Ben Foden - wearing "light" products but she was upset by the criticism because she'd only used the cosmetics to cheer up the tot after she'd broken her arm.
Appearing on UK TV show 'Loose Women' on Tuesday (15.05.18), she said: "She had an accident at home, she had a nightmare and whatever she fell on she broke her arm.
"It was awful. That weekend in a way to cheer her up...
She couldn't even use the swimming pool or anything, it was raining outside.
"We went out for dinner and when we came back, she loves watching me put my make-up on and like every little girl she likes to dress up.
"She said, 'Can you just put a couple of little flicks on me.'
"So I did and I thought she looked really cute and it was a special moment and she was really happy.
"So I put it on and we all share our lives on Instagram and I didn't think it was a problem, but the keyboard warriors came out in force and were just saying, 'Shame on you', and all this stuff."
Panellist Ruth Langsford asked if it was mainly mothers hitting out and Una replied: "Yeah it was actually. I just think you look after your own and I'll look after mine."
Janet Street-Porter then suggested people have to "grow a thick skin" when posting stuff online, but Una insisted she'd never received a response like it.
She replied: "Yes, but I put stuff up all the time and I never get any negativity like that.
"It was this picture in particular. They latched onto the fact I'd put make-up on her, but it was quite light make-up.
"It was just dressing up, she has her own little bag of make-up that she puts on herself.
"It's like history repeating itself really, when I was young I used to dig my nails into my mum's lipsticks."
The 'All Fired Up' hitmaker - who also has three-year-old son Tadhg with her rugby player spouse - ended up keeping the photo on her account and blocked out trolls by removing the ability to comment on the post.
She added: "I just disabled the comments because I felt like that was a really special moment which was fun, and she was happy."