Hello
Yes you have a right to be concerned. The crew should have brought you a baby belt on the first request. Myself as a purser the way I play things with babies during boarding is by having the appropriate number of baby belts hanging from a bulk head by me. When a pax boards with a baby, I then hand them one of the belts that I have ready - this also enables me to keep track of when all the babies have been boarded.
If the baby was not secured with a baby belt before take off (in my airline) the cabin would not be secure. One of our fifteen points of cabin secure that the cabin crew have to follow is "Infants on passenger laps wearing infant/extension seat belts".
If it happens again, I would sugest that when the purser or whoever carries out the headcount that you ask at that point, failing that keep pressing that call bell until you get one.
With regards to your final point, in my airline at least if the aircraft is lined up on the runway and the engines are spooling up, the cabin crew are not permitted to make contact with the flight crew. There are 2 critical phases of flight, which are take off and landing. The take off phase of flight is the "Period between engine power being applied and the retraction of the undercarriage" - so basically if something happened at this point, the cabin crew would not be permitted to contact the flight crew and the flight crew would more than likely not take an interphone call from the cabin if the cabin crew were to attempt one. The pilots in all airlines are well trained in dealing with emergencies, therefore the safest course of action at times would be to continue the take off and come back round for an emergency/precautionary landing rather than to reject the take off and risk a runway excursion.