Call each of the children and explain the situation that in the past it was no problem to store their things, but you are now organzing and have need for the space. Could you please get those moved to your place in the next two months? That is a reasonable timeframe and you are delivering the message in a gentle manner. You might get even prompter action if you state that you are using the space to make a guest room, or you now will be storing the bikes and the patio furniture inside for the winter. Also, let them know that you will be happy to donate any of the items they don’t want, or even add them to your trash if they are not of use to anyone. If the kids are out of town, offer to ship items they wish to keep. Go over with them by phone what you believe is theirs so you aren’t shipping something they are just going to give away or trash when it arrives! You can even email photos if there is something they don’t remember. If all this fails, let them know you are clearing out their stuff into a storage unit, and they can either get it from there, or they can pay the rent on the unit after the two month period is over. Set a deadline—that is important.