Hello, im new but just wanted to say I think you are brilliant, its really hard to read your hurt and not be able to help
Also you are doing a great job dealing with how you are feeling AND not letting it affect your children (which is the impression im getting). They both sound like beautiful children and you should be really proud of them and yourself.
As others have said, its really early days, please dont be too hard on yourself and I think if people close to you knew how much you needed them, they would be with you, sometimes the most difficult thing to do is let people in RL know how much we need help (and accept it).
As for the party, its a lovely idea, dont put too much pressure on yourself though, children prob wont even notice details, so long as they have fun and maybe get a few sweets - you could wrap them in little parcels (like a present) instead of gift bags?? A cake can be easy from Tesco/Asda etc, you can get them with a photo on, or character, etc.
When the party is over, maybe you & the children could have a balloon each, put a message on them and watch them float up to Daddy. I dont think them remembering Daddy is morbid at all, it is prob helping them cope in their own way.
Sorry ive gone on but if I can help in any way, let me know (im from oop north but still here if I can help!) x x