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There is a link on Thomsons website on the Seats with Extra Space explaining their policy.

"Seats with Extra Space

These include seats that are close to, or next to, emergency exit doors, behind a bulkhead, or behind a dividing wall. If you fly in one of our 787 Dreamliners, they may also be located in the first 4 rows of the Economy Club cabin. Seats with Extra Space are only available for adults and children 14 years and over to book in advance, but we may allocate these seats to families with children if the seats are not located in an emergency exit row. Emergency exit row seats are only available to passengers who are able to move quickly and operate the emergency exit without difficultly if necessary in line with European Aviation Safety Authority (EASA) regulations. Just a few things to bear in mind before you book an emergency exit row seat.

Any passengers who because of…:
- physical or mental impairment or disability
- age or sickness
- physical size
- have difficulty moving quickly cannot sit in these seats.

Neither can anyone…
- under the age of 14 (whether accompanied or not)
- who requires an extension seatbelt
- under the influence of alcohol
- travelling with an assistance animal.

These regulations have been put in place by the European Aviation Safety Authority (EASA) and are for the safety of our passengers. The cabin crew have the final say and we kindly ask that their decisions are respected. "