Separate Assessment and Children's Allowance

Apologies if this question has been asked already.
With the talk of Children's Allowance being taxed or means tested in the next budget, has anyone any opinion in the case of a single income family of 5 (3 kids).

It has been suggested that by opting for separate assessment, any means testing or taxation of the allowance will apply to the recipient (usually the non wage earner) and therefore any increased tax on the wage earner will be more than offset by the retention of, or limited taxation of, the Children's Allowance by the non wage earner.

If this is the case, at what stage can you/must you opt for separate assessment - can this be left to the end of the tax year?

Separate assessment? or separate treatment?
In separate treatment, there is no transfer of unused credits / allowances. That can be selected before the end of the tax year ( calendar year).

Separate assessment must be requested before 1st April in the tax year, too late already for 2009.