Zombie WISH had seen this yesterday as may have been of more use, but will post now anyway as - assuming you have told them today - there is still lots yet to do and which may help them. Will try to be succinct, but can't not start by offering the hugest of condolences and sympathy, not just for your DC's but also for you as you are unquestionably in for a very rocky road.
My Mother died 4 weeks ago tomorrow; aside from my own feelings as losing her, also had (still have, obv) DC's who lost their really beloved Nanny to try and take care of (mean in psychological sense; bereavement sense; as well as all other obvious ways). So here is my best advice:
1: With a DS of 15 and DD of 11, they will both likely react very differently - not just as point told, but also subsequently. Agree with ALL who have pointed you to Winston's Wishes and if you haven't been able to go to their site today, PLEASE do asap tomm or get a trusted family member or friend to do it for you so that you DO have benefit of WW guidance.
2: Here are the relevant CRUSE pages (have literally just cut/pasted them from mail to school, so apols if not in 'right' order).
www.crusebereavementcare.org.uk/CYPPerspective.html - General advice for schools when a pupil is bereaved.
www.crusebereavementcare.org.uk/SchoolsRefusal.html - School refusal
www.crusebereavementcare.org.uk/CYPAdolescent.html - for your 15 year old.
3: Won't waste your time/space here by recounting all my experiences/learnings from past cpl months (my Mumma was terminally ill so knew she was going before she did then die), will just try and bullet most useful:
- Vis initial reactions, 15 year son will almost certainly be profoundly angry (see CRUSE thing above); 11 year old daughter will almost certainly just be be bereft (how big a part in their lives was their Mother, as that too will have an effect on impact on all?).
- Strongly advise you do NOT lie to them (other than the - even if not true - that she died pain-free, as THAT is something they will go over and over in their heads and be beyond upset by). Re ALL else, there is no other safe/healthy option in L/Term but to just be very honest with them. Ergo, if you have 'fudged' stuff today, would really suggest correct it tomorrow and in context of you were simply trying to do the 'right' thing in innately wanting to protect them, but realise that they do need to know the truth.
- Re not seeing her before died - even if wired up to machines and distressing - you can't undo this now (& I KNOW how hard it is to have to make quick decisions when in such utter shock - if search my posts, will become clearer but without taking up space here); but they should have had the choice and been told (am so NOT criticising you here, just passing on all I do now - sadly - know).
- Honesty - however painful - is the ONLY 'right' thing you can do. So if you think you fucked up by NOT telling them, then simply explain that fact, along with the shock you were in and the 'advice' from the hospital you got at time.
- Best options NOW are to allow them to have as much a part as possible in both 'choosing' her funeral and taking part in it if they want to (& do NOT let anyone of 'older' generation 'persuade you differently); and equally, let them not it they don't want to. [look at my profile as only pics on there are those pertaining to my Mumma's funeral - to & for the AWESOME MN'ers who helped me so much - and the really personal/'her' things we did for her funeral that helped my DS's HUGELY.
- I wouldn't suggest a 'delayed' 'celebrating' birthday from today to tomorrow, as they will not be in place to celebrate anything (& may, esp DS, be in a very angry place anyway). Would suggest that you 'frame' it differently at some subsequent point, IE "don't think of my birthday next year as sad; it's still my birthday but will also be the day we look back at Mum and remember her as she would want you to". Can combine that thought/framing with help from Winstons Wishes who should be able to help them not just now, but with summer camp weeks solely for people (think sometimes people 'forget' DCs are people too IYKWIM?) in their position - and those WILL be the ONLY other people they can relate to. Is combo of fun stuff; talking about lost parent stuff; remembering that parent (by Memory Box, communal balloon's up to sky; letter written 'to' that lost parent).
Also be aware that they may not want to talk to you about how feeling (for variety of reasons) so PLEASE don't take that personally but DO have a standby list of people you know and trust, and who they know and care for and trust. VITALLY, they will (ESP 11year old) - even if not expressing it - be scared may lose you now that mortality has walloped them in face. Without being uber overt about it, try to weave into conversations that they don't need to be - IE in explaining how desperately sad and unusual it is that their Mummy has died so young; by booking a holiday for ages away (IE stuff that simply per se tells them YOU WILL be around, but without shoving blunt conversations about that in when they may not be up to dealing with it, but simultaneously WILL need to be reassured that you are not 'going to go too".
Just realised how long this is. Apols, but having both just lost MY Mum (though obviously older than them so very different) AND having to deal with DC's grief (albeit for a grandmother rather than their Mother), am very acutely aware of all/both sets of feelings and issues.
Wish you all so much love and strength and would be doing you a massive disservice if said you won't need both. Un-MN'y hugs Zombie xxx