An investigation by the National Audit Office (NAO) found that Trowler stood down as a director of Morning Lane Associates and sold her shares in the organisation prior to taking up her new post in September 2013.
Morning Lane, which did not have any government contracts at the time of Trowler standing down, subsequently secured four Department for Education contracts, two as part of consortia involving other organisations, worth a total of £2.9m.
Although she did not take decisions relating to the award of contracts, Trowler, who the report states had a "close and personal" relationship with the sole remaining director at Morning Lane, was involved in discussions that resulted in a £1.2m contract extension in relation to work on the social work accreditation system that the firm benefitted from.
The report states that Trowler initially withdrew from the process of assessing bids for developing a "proof of concept" accreditation system once it became clear Morning Lane Associates were part of a bidding consortium also involving KPMG.
However, the report adds that Trowler did become involved in discussions about the proposed system after the KPMG consortium had been identified as preferred bidders.
After Trowler raised concerns that the proposal did not include an element of observation of social work practice, DfE officials organised a meeting with KPMG and Morning Lane Associates, which she attended.
"Following this conversation, officials agreed with KPMG that it would commission an independent review of assessment approaches to determine whether an element of observation of social work practice was necessary," the report states.
As a result of the independent review findings, the DfE negotiated a contract extension to include an element of observed simulated practice in the assessment process.
The original contract, agreed in March 2015 for £2.6m, increased to £3.8m in October 2015 to incorporate the additional work.
The report states that the DfE acknowledges that it could have dealt with the situation differently to avoid a perceived conflict of interest.
"The department told us that the decision to consult the chief social worker after the competitive procurement process was taken in order to ensure the strongest possible assessment system and thereby ensure maximum value for money," it states.
"It has also told us that it considers the results of the ‘proof of concept' work show this was achieved.
"The department recognises that alternative approaches might have been taken to more fully manage any perception of a conflict - for example, seeking the views of other social work practice leaders, rather than the chief social worker, on the preferred bid."
Although the NAO report states that Trowler raised the potential for a conflict of interest in relation to a Morning Lane bid for innovation fund cash, and did not take part in relevant discussions, she did participate in considering other bids for the cash-limited programme.
The NAO also said it could find no record that Trowler had been asked to complete an assurance framework record - an annual return outlining any actual or perceived conflicts of interest - since her appointment in September 2013, despite this being DfE policy.
The DfE told the NAO that a review of departmental records showed that, as a result of an error, "a very small number" of senior civil servants, without budget or management responsibilities, were not asked to complete the document.
This included the chief social worker and some policy advisers. The DfE said it is taking action to ensure the error does not happen again.
Morning Lane has won a total of four contracts since Trowler has been chief social worker for children and families.
In February 2015, the DfE awarded its first contract to Morning Lane to act as "improvement adviser" for two years, worth up to £120,000.
Then in March 2015, Morning Lane received £1.7m to test a new social work approach through the DfE's £100m social care innovation fund.
Morning Lane also received a share of £3.8m that was paid to a consortium led by KPMG to develop knowledge and skills statements and tests for social workers.
It was also awarded a second improvement adviser contract worth up to £264,000 in August 2015.
A Department for Education spokesperson said: "The chief social worker has no financial connections to Morning Lane Associates and, as this report shows, she has always taken the appropriate steps to recuse herself from discussions involving the company.
"The report highlights a number of areas where the department's processes could be more robust and we are reviewing our processes in light of this report."