toilet will not flush

[dbc3269]

water fills im bowl, flapper will not close seems to be air coming back into flapper which holds flapper open, does not appear to be clogged vent pipe?? It is the kids toilet who knows what has been flushed,

[Plumber2000]

Turn down the water flow at shut off, if water is entering tank too rapid, this can cause flapper to not seat back down.