I would avoid making one adult/one child car journeys the normal solution.
The FA’s travel guidance is built around planning transport and appropriate safeguards. If a child needs help getting to training, I would first look for an arrangement involving their parent/carer, another family or a planned club transport solution.
If a one-to-one journey genuinely cannot be avoided, the club’s welfare policy should tell you how that situation is managed rather than the coach deciding alone.
The parent should know exactly what is happening, and the arrangement should be transparent.
This is not about suggesting the coach has done anything wrong.
Boundaries are useful precisely because they protect well-intentioned adults and children from avoidable situations.
If the request is becoming regular, that is a sign the club needs a proper transport plan rather than a series of favours arranged privately between one coach and one family.