You need to talk to both sets of parents before you set anything down.
If you are intent on leaving care to your DM, then I'd advocate leaving your money in trust for your children and have that trust adminstered by your MIL, also to have her as backup carer if DM is unwilling or unable to oblige at that point.
That said, are there no godparents around? These are the people we chose to have the best interest of the children throughout their life, and at that point they understood they might have to step up if tragedy occurred. Primary point being that they are age-appropriate for your children and are statistically more likely to still be around in 10 years time.
Lastly, you need to determine how the money should be assigned to your children, to ensure fairness, and aid whomever is administering the trust for them. Should they reserve money for tuition fees, or house deposit, or both - or should the money be permanently pooled (and so one can lose out to the others needs). Should you be holding back money for their discretionary spending at age 18/21, in the way you might have gifted them on those birthdays, etc?
Do you expect the children to stay locally, in your now paid-for house, or for them to move in with the carers? If so, you need to consider whether that changes your views on who cares for them - geography may no longer be an important factor.
Either way - much of this should not go into your will but into trust and other documents that indicate your wishes (ultimately, they will not be binding instructions).
Forget the siblings; no self-respecting brother/sister would expect to be put ahead of 2x orphaned children, but you make no mention of them in your children's future life - so perhaps have a think about what you expect/desire from them if tragedy strikes. Are you asking any of them to execute the will?