This is a bit of a trap long term. What you end up with is one child getting Christmases totally planned around them, and twice the fun. While younger children don't have Christmas unless SDC are there. It's a damaging message that they aren't enough on their own.
Your younger children need to know that they are worth the effort and deserve the fun whether SDC are there or not.
Also if you start moving Easter now, it might be hurtful to SDC if you realise later that DD is old enough to notice and go back to having Easter at the right time. Better to start as you mean to go on.
I am speaking from experience, I used to just do Christmas when DSD was here, but as they all got older it got very obvious my children's self esteem was being impacted and they felt like we couldn't have 'special' time without SD. While DSD was basically becoming quite entitled as everything was planned around her at both households.
Now we do a 'christmas period' type thing,. We have Christmas day with whoever is there on the day. Then when SD comes (if she wasn't there on the day) we have another nice day when she has her presents, plus we save a few of the presents from the extended family for them all to open together. Which is normal as we have a couple of visitors over the Christmas week so presents tend to be a bit spread out anyway. But it's not a replacement for Christmas day. It doesn't have to be as she has that at mum's!
One other thing to note is that this sort of thing will also be hard for younger children. Mine missed their sibling so much when she wasn't with them, as we have 50/50. So it is important that their life when she is not here is kept as stable as possible.
I do understand that you feel sad as you are used to having DSC. But as a parent you have to get yourself used to sometimes being a family with younger children only. It is not fair on them to mope when DSC are not there, DSC are doing something special with their mum and your DC deserve the same.
I realise this is Easter not Christmas, but am giving you my Christmas example is it is another special date.