I have a 1 and a 3 year old and my husband is not Catholic and does not attend Mass with us. It is almost always a challenge. I decided that I want my 3 year old to understand that Mass is not just another place in which she can do what she always does - play with toys, be loud, eat snacks, drink, etc. I don't want her laying in the pew, asking to go to the bathroom every Mass or thinking it is OK to be disruptive by talking, yelling, etc. This is a tall order because she is little. But I'd rather set the bar high now and have her learn that this is a participatory service and that other people are trying to focus and participate in the liturgy and that she can disrupt them and it is rude to do so.
So, with all that said I've made compromises. At 3 (almost 4) she IS capable of being quiet and non disruptive. I know other parents would say that is too much to ask, but she can do it and I do think it is because I haven't given her excuses to think otherwise. We do pack a small bag of picture books about Jesus, a Mary doll I found on Etsy, and add other various things in from time to time. I don't always expect her to stand up, kneel but she has to sit quietly.
Just having the expectation in place and trying to help her understand reverence on a 3 year old level has really helped us in the last few months. I do try and attend a Mass on Sunday in which there is children's church so she can get a 20 minute period of interaction with peers and age appropriate religious education, and that has helped as well.
I also think you shouldn't stress too much about it. There is *always* a kid at Mass who is going to have a melt down, a baby who is crying because they are tired/hungry/overstimulated, etc. Our church has a lot of young families and there is always a constant rumble in the congregation from their fidgeting and outbursts. Everyone seems to be very understanding and I've almost always had the people sitting around/behind me "help" me when my kids get out of hand by distracting them from their upset by playing a quick peek a boo or something.
My baby is MUCH harder to deal with. He wants to crawl and touch and see everything. He is too little to expect him to be able to make it through the service quietly without a lot of help - toys, cheerios, cup of water, playsilks, etc. are always in the bag and I just go with the flow trying different things to help keep him quiet. I generally sit with friends so if I have to walk him out for a bit because he is too loud my friends are there for my daughter to sit with.
Anyway, this was a long way of saying that I don't think anyone attending Mass expects small children to be perfectly quiet and still throughout the service. Do your best and don't let them just be wild. That actually makes me a little crazy when I see parents who just let their kids jump up and down, bang toys, and create an elaborate picnic for them in the pews - I really think most 3+ year olds can get through Mass without a peanut butter sandwich and 4 trips to the bathroom.
