if you have ILR when you apply for the visa for the child, the child will be granted ILE Indefinite Leave to Enter ( same as ILR)

if you apply for a settlement visa for the child before you have ILR, the child will be granted the same visa as you have, and once you can apply for ILR you should be able to apply for ILR for the child at the same time. it doesn't matter how long the child has been in the UK.but doing it this way means you have to apply for and pay for an extra visa (settlement visa + ILR) while if you had ILR its just one visa (settlement with ILE)

thou the problem is the longer you leave applying for the visa, it could be more difficult to prove you have 'sole responsibility'