Paragraph 1 of Schedule 2 CA 1989 provides that every local authority shall take reasonable steps to identify the extent to which there are children in need within their area.
In R (G) v Barnet LBC  3 WLR 1194, the House of Lords held that, although section 17 does not impose a mandatory duty on a local authority to take steps to satisfy the needs of a child in need, a local authority is under a duty to assess the needs of a child who appears to be in need - , , .
Para 77 - LORD HOPE OF CRAIGHEAD:
My noble and learned friend Lord Nicholls has said that, on the respondents' approach to the construction of section 17(1), it follows that a local authority is not under a duty to assess the needs of a child in need under section 17(1) and that this would go far to stultify the purpose of Part III of the Act. I should make it clear, before I embark on my analysis, that I am unable to agree that this conclusion follows from the respondents' argument. Section 17(2) provides that, for the purpose of facilitating the discharge of the general duty under that section, every local authority shall have the specific duties and powers set out in Part I of Schedule 2. The duty of the local authority to take reasonable steps to identify the extent to which there are children in need in their area is to be found in paragraph 1 of the Schedule. That will involve assessing the needs of each child who is found to be in need in their area as paragraph 3 makes clear.
Para 110 Lord Millett
It does not follow that the social services authority is not obliged to assess the needs of the individual child. The existence of a power to provide assistance to a class involves a duty to consider whether a particular individual is eligible for such assistance; and in the present context that involves assessing the needs of the child in order to decide whether and the extent to which the authority will meet his needs. But there is no need to invoke this implied duty; as my noble and learned friend Lord Hope has explained, the relevant duty is expressly provided by paragraphs 1 and 3 of Schedule 2.
Para 135 Lord Scott
In my opinion, in agreement with my noble and learned friend Lord Hope of Craighead, section 17(1) does not impose a mandatory duty on a local authority to take specific steps to satisfy the assessed needs of a child in need. If a mandatory order against a local authority to take some specific step is sought the applicant must either point to a specific duty to take the step imposed elsewhere in the Act (or in other legislation) or must invalidate the local authority's decision to decline to take the step on the usual reasonableness and proportionality grounds available in judicial review challenges. In my opinion, the appellants' appeals, in so far as they are based on a mandatory duty arising under section 17(1), should be dismissed. I am in full and respectful agreement with the reasons given by my noble and learned friend Lord Hope of Craighead on this issue.
The duty to assess is triggered by the mere physical presence of the child with the authority’s borough and nothing more is required. Accordingly, the duty may be owed by more than one authority simultaneously and will be owed by the authority in which the children live, together with the authority in which the children attend school: Sandra Stewart v The London Borough of Wandsworth, The London Borough of Hammersmith and Fulham, The London Borough of Lambeth  EWHC Admin 709