My family and I travel overseas a lot. Shortest flight over 17 hours. They do great with minor adjustments based on age. Have new cheap toys for them to open (matchbox or hot wheels cars, figurines non choking size for your age group, duplos or legos for older age). Be sure to bring a snack or granola bar they like and can eat. Let them be free in seats, even play on floor in your row, without bothering other travelers, once the plane is in the air (my 4 year old knows to walk on her own to the flight attendant and ask for water if she is thirsty). They usually cry from ear pain at take off and landing. Have a drink available (pick up in airport) or like safe food based on age (gum, gummy bears or things to suck on like lollipop). For beach, just need water safety devices or jackets, sunscreen and (and bug spray if applicable). Have hats and coverups in case sun gets too much. Both my kids had thin full body wetsuits. And just enjoy them at this age.
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