psql trigger for update in single table field

I have two simple tables:

CREATE TABLE nene(
id integer  primary key,
nombre character varying,
altaCasal integer);

CREATE TABLE actividades (
nen integer,
actividad character varying,
CONSTRAINT nen_fkey FOREIGN KEY (nen)
REFERENCES public.nene (id) MATCH SIMPLE
ON UPDATE NO ACTION ON DELETE NO ACTION);

and I want to make a trigger that when I insert something in actividades I do a update in nene in the field altaActividad indicating the number of activities to which a child is pointing, I have generated a function and its trigger the problem is that when I do a insert I update all the fields, how can I just update the id of the child to whom the insert is made?

CREATE OR REPLACE FUNCTION public.registre_actividades()
  RETURNS trigger AS
$BODY$
declare
acti integer=0;
begin

IF (TG_OP = 'INSERT') THEN

update nene set altaCasal=(acti+1);
RETURN NEW;
        END IF;
        RETURN NULL; -- result is ignored since this is an AFTER trigger
    END;

$BODY$
  LANGUAGE plpgsql VOLATILE
  COST 100;

CREATE TRIGGER Acti after INSERT ON actividades FOR EACH ROW EXECUTE PROCEDURE public.registre_actividades();