Warning: this story contains photos that may be upsetting, but it's important to warn parents about the risks associated with black henna tattoos.

When parents are preparing for a family vacation, they plan to entertain bored children and wait in long lines.

Almost no one expects that their children will be seriously injured, especially by a fun and innocent activity at their hotel.

After stylish henna "tattoos" burned their daughter, the Gullivers are warning other parents to avoid this popular trend.

"The next morning the whole tattoo was starting to get itchy."

Martin and Sylvia Gulliver took their children, nine-year-old Sebastian and seven-year-old Madison, on a holiday to sunny Egypt.

What was supposed to be a relaxing trip became stressful when Sylvia was rushed to the hospital because of an issue with her gallbladder.

But the children were so well-behaved by their mother's side during the two days she spent in hospital that the Gullivers decided to reward them.

They paid for their hotel's salon to give both children stylish temporary henna tattoos, which are popular with young children everywhere.

Madison was happy with her new ink, until she woke up the next day with an itchy arm.

"We noticed there was a small patch on the top of the tattoo that was raised but we couldn't see any redness," her father Martin remembered.

"The next morning the whole tattoo was starting to get itchy, so we washed it off which revealed a rash in the outline of the tattoo."

The family had no idea, but this type of reaction is actually very common, and very serious.

"They were trying different things but it was getting worse and worse."

The dyes used to make brown or reddish henna tattoos are natural, and safe for most people.

But the distinctive black henna tattoos get their color from a chemical called paraphenylenediamine, or PPD.

It's used in plenty of cosmetics, including hair dye, but it's also known for causing strong allergic reactions, especially in young children.

The risk of burns from black henna is so serious the FDA has actually released warnings about these tattoos.

By the time the Gullivers returned home from their vacation, Madison's rash had turned to blisters.

She was treated with steroid creams and ointments, but was eventually referred to a burn specialist.

"We started to panic," Martin said.

"They had never seen it before, and each time we went in they were trying different things but it was getting worse and worse."

Soon, Madison was being treated for chemical burns on her arms, and doctors had to cut away the top layer of blisters to treat them.

"We want to get the message out to other people about this."

Madison is recovering from her burns, but her arm may be permanently scarred from her ordeal.

There are other long-lasting side effects from these reactions too:

PPD reactions can leave patients sensitive to penicillin and anesthetic drugs, and Madison seems to have developed a penicillin allergy.

"We were entirely unaware of the dangers," Martin said, and he warns other parents not to make the same mistake.

Even at a professional salon, black henna can cause painful and serious reactions.

Be especially careful when traveling overseas, because the use of PPD may not be regulated.

The hotel where Madison got her tattoo has since apologized and is not offering henna tattoos anymore.

"I think it's partly my fault because I didn't know about it," Martin says.

"But also the fault of the salon because they are using dangerous chemicals on children."

"We want to get the message out to other people about this."

Let's make sure every parent sees this important warning!

Here are more health warnings you should see:

• 7 early warning signs of a heart attack that most women ignore.
• Scented candles are a serious risk to your baby's health.
• 13 cancer symptoms you can't afford to ignore.

[H/T: Good Housekeeping, Metro, The Sun]