well here a way to do without simpe all you need is moveobjects cheat and insimenator  You need a child acting like toddler ? So  make a toddler sim then with time cntroling device gow him up to a child then ctrl shift c moveobjects on. Now in buy mode you can take your sim like any other object, take him (or her) and delete. Now go to the town screen DOn'T FORGET TO SAVE CHANGES!!! No just go back to the family et Voila! crible crable bums your sim is back but now he is child but acts like todller. One big problem when walking he is flying  but if it dosent  matters here you are the way to do it whisout any external program.