Since their movement is inevitable, why don't you throw out a grand farewell party for your neighbor friends and kids so that you will be able to part ways amicably and they will be in contact with you even though they are in some other place. Plan the said party  at least a week before moving out. That way you could still make some adjustments in case they are going to get busy. Schedule it during the weekend so that other neighbors and friends can also be invited. Call your other neighbors who formed a tie with your friend and together in your kids outdoor furniture, make a list of what you should do.

The first consideration is the venue. Where do you plan to hold the party? You could suggest your place and make it bigger by arranging the kids furniture on the sides so that you will have a bigger space. Another is you can make it on the garden so that you can breathe in fresh air. It really depends on you and your friends where you will hold the party. You could even surprise your friend at their house but that would not be advisable because by that time, they might have already packed their stuffs.

Then plan about the food. You don't need to spend much for the farewell party. All of you could just have a potluck so that everyone can participate and bring their own food. You could prepare food for the small kids. The food is dependent on the time you will hold the party. It may be during lunch or dinner or probably during snacks so that the kids are around the house. You could place some finger foods on the childrens furniture so that the kids will have a place of their own while the adults are on the couches.