Maddie was eating a Happy Meal her mother Laura Arber, 32, bought from the Aldershot, Hampshire, branch of the fast food giant on Tuesday.
She managed to get the chicken nugget out of her daughter’s mouth and said: “It was a mask, it was absolutely baked into it”.
McDonald’s said it is investigating.
Ms Arber told the BBC: “I had to put my finger in her mouth to make her sick and it came up all speckled with blue.
‘Disgusting’
“I couldn’t work out what it was but I looked at the box of nuggets and could see something blue sticking out of another one.
“It was a mask, it was absolutely baked into it, it had gone like chewing gum. It was disgusting.
“If I hadn’t been in the room I just don’t know what could have happened.”
The mother of four said she went straight back to the restaurant to speak to the manager who told her the nuggets were not cooked on the premises.
Ms Arber said she and her daughter have been put off McDonald’s “for life”.
She said she wants to make other parents aware, adding: “Just because it says it’s a Happy Meal doesn’t mean it’s safe.”
McDonald’s said food safety is of the “utmost importance to us” and said the company places great emphasis on quality control, following “rigorous standards to avoid any imperfections”.
A spokesperson said: “As soon as we were made aware of the issue we opened a full investigation with the relevant supplier, and have taken action to ensure any product from this batch is removed from restaurants.
“We would like to offer a full apology to the customer in question and understand they are currently in conversation with our customer services team.”