The mother of 5-year-old "Toddlers & Tiaras" star Isabella Barrett doesn't care that her daughter's performing LMFAO's "Sexy and I Know It" in nightclubs -- telling TMZ, "That's a real catchy song!"

Susanna Barrett was out shopping with her daughter yesterday in NYC -- telling us, Isabella's a big fan of top 40 music ... and that's why she was able to spout off the LMFAO lyrics at a New York nightclub this week. According to Susanna, Isabella also loves Lady Gaga and Rihanna.

But Susanna quickly pointed out ... there are limits -- for instance, Isabella has never seen the music video for "Sexy and I Know It" ... which shows the lead singer shaking his man-package around in a Speedo (below).



All in all, Susanna insists the nightclub experience was pretty tame -- "We went for a pet charity event ... We left by 9 o'clock."

She's just 5 years old, but "Toddlers & Tiaras" star Isabella Barrett is already making nightclub appearances -- singing songs about her sex appeal -- and TMZ has obtained video evidence.

Barrett graced Libation NYC with her presence Monday night for a special charity fundraising event -- where she danced and sang along to LMFAO's "Sexy and I Know It."

In case you forgot -- Isabella is the same girl who said her fellow "Toddlers" star Paisley Dickey dresses like "a hooker."

Sure, the video's pretty cute -- but when we were six ... we sang songs from "Barney & Friends."