Hi new-yorkshire. I am in a similar situation with two step-children, but I have just applied for and received my ILR/ SET(M). When I was applying for FLR(M) in 2017 I struggled to find any info/ anyone in a similar situation online so I will add my 2 cents even though it does not match your situation entirely. So forgive me, there might be a bit of rambling here!
The paper form that was previously used for FLR(M) did require information about the sponsor's children even if they were British Citizens so it sounds that in the online form that is still the case (I have not seen it though). When I filled the paper FLR(M) form I did have to include proof of their citizenship - copies of the kid's birth certificates, their passport pages but no proof of where they resided like NHS letter etc.
The kids live primarily with their mum and stay with us 1 night a week and every other weekend. So would consider her address their primary residence. I wrote this out in the additional info/cover letter (can't remember exactly but it was written out clearly). As far as I am aware the Home Office is only concerned that your partner/ you make the £18,600 to cover YOU and they don't take into consideration the sponsor's previous relationships and British Citizen children even if they pay child maintenance (as my husband does pay this). There is no official maintenance arrangement in place for my step-children either as the split was amicable and my husband pays monthly more than what was recommended by the official calculator for this.
You might be pleased to know that the new online ILR / SET (M) form asked for more limited info than the FLR(M) form did. I read that this online form is based on algorithms that might tailor the questions depending on whatever info it is linking to in background (previous info they collected from me from FLR (M)? who knows!) it asked:
1. Do you and your partner have any children together?
2. Do you have any children whose parent is not your partner?
3. Does your partner have any children living in the UK from a previous relationship?
4. Provide details of where the children live, who they live with and their relationship with that person. Include the reason they do not live with your partner.
5. Are any of these children subject to immigration control?
I did again provide copies of their birth certificates even though not asked for and I wrote in the form for no. 4- that the kids live
primarily with their mum and that they live with us 1 weekday and every other weekend, wrote their names out and dates of birth and stated that they were British Citizens. That was it.
For your FLR(M) q's-
In terms of the "new" FLR(M), what documents will I need to provide for the kids given our situation? Also, I'm now unsure when answering questions like:
Do the children live with you? Half the time? Every 2 days? 6 months per year?
Who is financially responsible for these children? Husband, Sally & Myself?
Is your sponsor financially responsible for anyone else? Is the answer to this "No" since the kids are covered earlier in this section?Again, I am not sure of your entire situation but if it were me doing this app again, I would state exactly how many days they stay with you a week. And I would also state that the parents are financially responsible for them as that is the case- if you were not in the picture it is down to them frankly. I would also answer NO to the sponsor being financially responsible for anyone else unless there is some kind of mandate in place for alimony?
I am sure someone else will come along with more info but I did fret over the same questions when I filled this out in 2017 and even just last week when I was double checking my ILR form!
best
