A fun travel souvenir for your kids is foreign currency. Bring home a few Euros, Pounds, Yen, Dirhams, etc. and help your kids start a collection. It’s a great learning project and it cleans out your wallet of the last of the foreign currency that would have been too expensive to convert back to your home currency.
A photo album or something with sleeves to hold the money works well for storing and documenting the currency collection. Label each coin with where it’s from and its denomination soon after your travels as trying to remember later is difficult. Often the writing on the coin is difficult to read.
Also teach your children about the value of the money and what it buys in its home country. For example, a $100 Hong Kong note was worth about $13 U.S. recently and that amount buys two Starbucks coffees and one bagel.