Any loves, obsessions, whatever they've got? Depending on budget, likes and dislikes you could think about anything ranging from going to a special loved movie together to going to a zoo, doing a beachtrip, picknick, seeing a loved musical, whatever you can think off. You can still give a gift and get to spend some one on one time, if you could manage to visit them long enough and/or have them over long enough to do such a thing.
Only limit is imagination. You can basically come up with anything for any budget that fits a childs likes, loves and desires.
For me those were the best and most dear gifts of all. It showed love, attention and time together.
Add; you could buy tickets or make a "voucher" yourself and have those put into the childrens stockings. For younger kids adding a calender on which they can count down the day also helps them to oversee the time they'ld have to wait to do ............. .