Safety officers say a 12-year-old boy had to be taken to hospital after a pitch invasion last Saturday.
Now they have reduced the numbers allowed into the ground from 10,800 to 4,649 with no standing. It will come into force for the low key semi-professional match against Cross Keys this weekend.
If the pitch ban is then obeyed Carmarthenshire's safety committee says it will look at restoring full capacity for the Scarlets game against Toulouse in the Heineken European Cup on 22 October.
While the Cross Keys game would be unlikely to attract more than 2,000 fans, the Toulouse match is arguably the club's biggest game of the season so far and the Scarlets would hope it is a sell-out.
The safety committee, made up of council officers and representatives from the police, fire and ambulance services, warned last week it would cut capacity if the ban was not observed.
But children ran onto the pitch again on Saturday and the committee says a boy was taken to West Wales General Hospital in Carmarthen with a neck injury.
Committee chairwoman Sue Watts said: 'Whenever Stradey Park is used for sporting events, the Scarlets and Llanelli RFC are bound by the terms and conditions of the ground safety certificate.
'We have been concerned about the risk involve and a child was actually injured and treated in hospital during the latest pitch invasion.
'The committee has now agreed to restrict the seating capacity at the match between Llanelli and Cross Keys on Saturday.
'The club will now be notified of the decision and if it is proved that access can be controlled and crowd management measures are taken, we will look at the issue again.'
Some fans have launched a petition against the half-time ban. Until this season kicked-off children had played on the pitch during the interval for as long as can be remembered.