One has to be very careful about telling children not to play with their favorite toy on Shabbos (Yeah I know many toys are electronic and assur) but yelling Muktah or Shabbos at young children who might not understand might give them a very negative view of shabbos and instead of a good day, they will see it in a very negative light
One should try to keep away the electronic toys first and worry about the lego’s and tinker toys later, Taking them all away and making the kid take out a sefer instead may backfire