Now the mums and dads have prepared a dossier to be sent to Children's Commissioner for Wales demanding actions.
Their accusations include:
• Children as young as four refused permission to go to the toilet unless they asked in Welsh;
• Parents told not to read to their children in English because it hampers them learning Welsh;
• A six-year-old boy was too afraid to speak English outside of school in case he got into trouble;
• A child heard speaking English told they were making Welsh 'die out';
• Another child told to 'move to England' if he wanted to speak English.
One mother was also urged to stop reading her child stories in English at bedtime, they say.
The parents, who have asked to remain anonymous, have set up a website, called BiLingo, and a campaign group in Ceredigion, a Welsh-speaking stronghold, and made a formal complaint to Keith Towler, Children's Commissioner for Wales.
A statement on the website says: "We fully support the principle of bilingual education and think equal importance should be given to English and Welsh.
"We want our children to have a strong sense of Welsh identity regardless of first language.
"We launched this site because we were concerned how the Language Strategy has been interpreted within Ceredigion schools and our intention was to assess how much concern there was amongst parents.
"It is clear there is concern, and that many parents do not feel comfortable going to the school directly.
"We have now drawn attention to the concerns which was our ultimate aim and feel this is as far as we should take it.
"We urge parents who are not happy and who are not comfortable going direct to their school to contact their councillor. We have been assured they will be able to hear your concerns in confidence."
Tory Welsh Assembly Member Suzy Davies said: "If true, these negative experiences can create a real sense of conflict for children and do nothing to promote the value of Welsh to families who may still be sceptical."
Around a fifth of the three million people in Wales speak Welsh.
Councillor Hag Harris, of Ceredigion Council, said: "We'll be happy to discuss any concerns."
Aberaeron councillor Elizabeth Evans told the local paper she was concerned about the severity of the allegations but a proper investigation could not proceed without those involved stepping forward.
She said: "I appreciate why parents would want anonymity if they have children that have been placed in such situations, but such allegations will be difficult to investigate without all the necessary details."
She said the group had now contacted all schools asking for a copy of their response to the consultation regarding Welsh language by Ceredigion council.
"I suppose the next question would be, if the schools haven't responded, why not?"
Suggested For You
SUBSCRIBE AND FOLLOW
Get top stories and blog posts emailed to me each day. Newsletters may offer personalized content or advertisements.Learn more