Children's commissioner Peter Clarke says the creation of a similar role in England, could confuse and undermine his position.
Peter Clarke was appointed in 2000
He said plans for a new post in England could mean he will not be able to act on behalf of Welsh children on some key issues.
He fears his English counterpart will have powers over non-devolved matters.
This means issues relating to the probation service for example, would be dealt with by England.
Speaking to BBC Wales' AM PM programme on Wednesday, he said: "It seems to me a complete nonsense to be honest and I'll be saying much more strong and informed things tomorrow when I've seen the details in full.
"The reason I'm worried about it is I try always to take the young person's perspective.
"Over three years now we've been trying to build up a system so that children understand that their commissioner is a Welsh Commissioner - and I'm there for them.
"Suddenly we are going to have to tell them - 'well, actually no, it's the English Commissioner you want for this thing or that thing'."
The post of Welsh children's commissioner - to respond to complaints from children - was created after a report by Sir Ronald Waterhouse in 2000 about the abuse of children over a period of 20 years across north Wales.
Two Acts of Parliament control the commissioner but he is independent from government.
Mr Clarke is also hopeful the new Children's Bill would be an opportunity to extend his power in protecting children's rights across a number of areas.
Plaid Cymru's Shadow Health Minister Rhodri Glyn Thomas AM said this could make it "impossible" for Mr Clarke to fulfil his duties.
"Much of his work could be undermined," he said. "It is a totally illogical situation.
"We have, here, yet another illustration of how flawed the devolved settlement in Wales is and how badly we need more devolved powers to ensure that ridiculous situations such as this cannot arise."
Children's Minister Margaret Hodge is due to launch the Government's Children's Bill on Thursday.