they get so much from other family members that we don't spend as much. Sometimes we will splurge on a family gift like 3 years ago when we bought the xbox 360 but we spend 50-75.00 for each kid and we usually get them a video game/accessory or movie, and clothes. They do get A LOT from other family members..(at least 4-500.00 each if you add their gifts all together)
We spend 100.00 each for their birthday and would rather spend more on their birthday to make their day more special. They get so much at Christmas that it actually gets overwhelming especially for little kids and they usually don't play with everything they get for a few months.
Determine the amount you want to spend by how many gifts they have received in the past. If you have a small family you may want to spend a little more than we do but I have a huge family and they get a 15-20.00 gift card from each family and my hubby's family spends a lot more because he has a smaller family and our 3 kids are the only grandkids, nieces and nephews on his side so they like to spend a lot on our kids for Christmas.
I do try to make sure they have the same amount of gifts to open. If they are gifts for all 3 of them to share I will write which kid should open that gift on the gift 1st. That way they get even amount to open but they do know that some are shared gifts. You can always grab some dollar store stuff and wrap them individually to make up the difference or separate outfits when wrapping them to even them out.
My MIL will send the Christmas money before Thanksgiving so we can get good deals on Black Friday so that helps a lot with getting the more expensive items cheaper and can usually get them a few nice outfits with the money we saved. I send her a list of what we buy with her money and she is always amazed at how many items we get. She doesn't live close to outlet malls or good shopping sales.