They claim the Government's nursery voucher scheme penalises four-year-old foster children because there is no clear procedure for giving them the Pounds 1,100 education voucher.
A Welsh Office spokeswoman said: "We recognise there are practical difficulties with children in foster homes. We have talked to social services and are continuing to consult them."
But a Swansea-based nursery action group is concerned that a separate procedure for giving foster children vouchers will discriminate against them.
Caroline Davies, an action group member, said that the problem arose because children in foster care did not receive child benefit - the voucher scheme's main database for identifying four-year-olds is the child benefit register.
Lesley Whitney, Norfolk's co-ordinator for services to children under eight, said her county encountered exactly the same problems as the Swansea nursery action group, but resolved difficulties after talks with the Department for Education and Employment.
Norfolk has now advised foster carers to apply direct for vouchers.
As very young children in foster care often had short-term placements it was important that the natural parents should have a say in where the vouchers were used, said Lesley Whitney.