Your situation mirrors ours almost exactly but 4 years ago. My DH started in Feb09 and DDs and I joined him in Aug09. However, we visited DH in April09 to have a look see, and during that time we visited the schools. Having seen what was on offer, only DESS fitted the bill for us. Luckily we got places for our DDs, but not until the June. I would apply to all the schools, but I really recommend coming and having a look to see which school(s) you might want to focus on.
I agree with louisea above. Definitely apply to DESS. Your 10 year old is very likely to get a place for year 6 in sept. Your younger one will then go to the top of the waiting list as a sibling (along with any other siblings). DESS will not know how many children in your younger DDs year will be leaving until sept fees are due which is around the beg of June. It's possible you will then get a place for her. My 2 DDs have been at the school for over 3 years now and we are extremely pleased.
I have heard that where DESS and DC are concerned tenacity and brown-nosing pays off! Just don't apply and then sit back and expect to hear a yes. You have to keep badgering and badgering.
Even if you send them somewhere else as a temporary measure it's worth keeping them on the waiting list for DC if you have your heart set on it, and keep phoning up OVER and OVER - I cannot stipulate this enough! The turnover of all the schools (even the very good ones) is very high here so you have to keep on the ball. It's about lucky timing.
DESS, DC, PH and Sherborne will all want to test your children and will charge for the testing and an application. When I phoned DC they told me my son's year group was so over-subscribed that there was no point in my even asking for a visit! They'll always happily take you money for the testing though. (They charge to test and to apply.) Compass do test, but it is more of a case of getting to know your child's needs/strengths to assess whether thay can accommodate them, rather than automatically looking for someone who will keep their results high. I have heard that kids who are really struggling at DC and Sherborne as left to flounder until they eventually leave or are asked to leave.
The DC tests (and Sherborne as well I think) can be done online or you can go out and visit and have them tested there, so they can look around and get a feel for the place. Although in the end you are lucky if you get to pick and choose based on your gut feeling. I think it very much a case of if you pass you are in and if you don't then tough - someone else who can pass, or who tested with higher marks that week will have that space. It's extremely competitive at most of the highly sought after schools, with the exception of Compass who are much, much more inclusive, but as you say - very expensive.
If your DH has been posted there by a British company you get priority at DC.
You are more likely to get a place easily at Compass because the senior school is still relatively new, and it has plenty of room for expansion. I think people are often a bit reluctant to try it for seniors as it doesn't have the solid track record in Doha that DC or PH as, and of course it's expensive so people are understandably nervous. They had their first cohort of IGCSE students last summer who did fantastically well, but then it was a very small group, so it's hard to know if the results were skewed by a handful of super-brainy kids. They got 100% A* at English though.
Compass is very bottom heavy with younger children. I think as those children come through, and they open their new senior campus in two years or so (allegedly) then Compass will be a big player to rival DC and Sherborne. Sherborne lost a lot of children across many year groups towards the end of last school year, but they seem to be sorting themselves out a bit now. Most people are happy with it, but it is very pushy and results driven - I know someone who teaches there and she finds it quite disappointing as she doesn't think they nurture the 'whole' child.
My child goes to Compass and honestly, I could not be more delighted with it. It is worth every penny in my book.
It might still be worthwhile applying for DESS. If your 10 year old is offered a place then the younger one goes on the sibling list and is bumped up the list. There is always a lot of momvement. People come and go at short notice. Also, once you are in the country you can call or drop in to the school easily. Not sure about DC as our kids went from DESS to DC. People are always saying that it is impossible to get in to DESS (or other schools) but someone must otherwise it wouldn't be difficult to start with.
Doha British School used to be called Doha Montessori School and has fairly recently changed its name. If you google that you might get more hits. As far as I am aware it is very highly regarded although I don't know that many people who go there.
I would recommend that you apply to Sherborne and Compass as well (both excellent but very, very different in ethos) as holding out for DC or DESS is very unwise indeed.
Some others to consider are Qatar Academy, American School of Doha (they do IB and IGCSE I believe) and the American Community School, which is very new in DOha, but the group has a good reputation globally. there is also a newish school called Loydence Academy which seems to be getting quite good reviews so far.
My DH is starting work in Doha next week, we are planning to join him in 6 months, and are hoping to enrol the kids (DDs 7 & 10) at the start of the new school year in Sept. We have applied to DESS, DC, Park House, and Doha British School. DC have told us no chance til Sept 2014. DESS have said maybe a place for DD 10 in Sept but not DD 7. Park House initially said nothing til 2014, then asked us to submit application just in case. Doha British School said maybe, so fingers crossed.
Just wondering if I should apply to any more. Sherborne and Compass are a bit out of our budget, but may be possible as a last resort. I don't know whether I should apply to them now, or only later in the year if we don't get places in the others. I am worried that once you do get a place in a school you have to accept almost immediately and pay the whopping deposit. I would hate to accept Compass, and then maybe get lucky and be offered a place at Doha British School.
BTW, what are your thoughts on Doha British School? I have not seen it mentioned much on other Doha school threads on this forum.
I really would be grateful for any input/advice/suggestions.