Martin Lewis has issued a warning to all families flying with Ryanair this summer.
The MoneySavingExpert.com founder explained how families travelling with a child under the age of 12 must pay for at least one seat reservation.
British Airways, Tui and Virgin Atlantic all guarantee that children under 12 will be seated with at least one adult from their booking without having to pay.
The cheapest standard seat reservation through Ryanair is £2.
Speaking on the Martin Lewis Money Show Live on ITV last night, he told viewers: "Most airlines will allow under 12s, some under 15s, to be guaranteed to sit next to one adult.
"Not Ryanair. It says the adult has to pay in order to have someone sitting next to them.
"Although to sit next to can legally be defined as 'separated by an aisle, or one row behind'.
"So generally, get your check-in as soon as possible, but don't be fear sold that you have to pay for it in other cases, it's just worth being on top of it to avoid that."
Martin also warned Brits to check their travel documents ahead of holidays to the likes of France, Italy, Greece, Spain and other hotspots this summer.
He warned that 5.4million European Health Insurance Cards (EHIC) are due to expire this year.
If yours is one of them, then you'll need to get it replaced for a GHIC which is "basically the same thing".
Holidaymakers also need to check their passport is valid.
Martin explained you need to check if your passport is under 10 years old on the day you enter many European countries.
You also need to check you have at least six months left on your passport from the day that you enter many destinations, including the United States and Australia.
For some EU countries, you need to have at least three months remaining.
If you need to renew your passport, you should get the ball rolling sooner rather than later, as there are currently huge delas.
Martin explained: "The Passport Office is saying allow up to ten weeks for renewals, but I'm hearing delays can be longer, and the Fast Track slots are limited.
"And let me be plain; an invalid passport, the travel insurance company will not allow you to claim if that's the reason you can't go on holiday."
