I am a first-year PhD student and I have two accepted publications as a first author since the start of my program. The thing is I thought I would be the one to travel to the conferences and present my work but my advisor is making the presentation, as he made me aware of it today. The conference is in Europe. He gives me an excuse that the reason he will be going is because he will be on the committee of reviewers for the same conference next year and this will be a good opportunity for him to know what is going on. The first conference is somewhere on the west coast and his co-PI also will be making that presentation. I feel a little frustrated that all my hard-work is being presented by people who had little or nothing to do with the hundreds of hours of work that led to these results. I feel this might be a trend that will continue throughout my PhD. My question: do you think I should challenge these folks or quietly accept their excuses for wanting to go, though unhappy about it as it is, and hope for a better deal in the future.
My question: do you think I should challenge these folks or quietly accept their excuses for wanting to go
Well, you've already asked about going, and the answer was "no." Nothing wrong with asking again, though.
However, I want to point out that the way your question reads, it sounds like your previous conversation was more about your PIs attending, not you. So, you should make it clear that you are interested in going. If you were to ask again, I would suggest asking something along the lines of "are there any funds available for me to attend the conference(s)?"
If this conversation does not change the outcome, you could always check with your institution to see if there are funds available for students to travel to and attend conferences. Some schools have these, some schools don't. You may be able to receive some partial funds and be required to cover the rest of the cost out of your own pocket if you really wanted to go and were willing/able to pay your way. In any case, it's a good idea to see if these funds are available to you.
though unhappy about it as it is, and hope for a better deal in the future.
No, hoping for a better deal in the future is not a good strategy. You need to have a chat with your advisor(s), and express your desire to attend the conferences where your work is presented. Again, as above, ask specifically about whether or not there are/will be any funds for you to attend conferences.
Depending on the answer, you may be satisfied that you will be taken care of the next time around. Or, you may find that at your school/department, students are more or less on their own in obtaining funds for attending conferences (at my PhD-granting institution/department, it was common for the PI to secure the funds for their students to attend conferences where their work was presented — your field/institution may be different).
In the end, if you are not able to come to an agreement about how your travel to conferences will be covered, it will be up to you to decide if your advisor's inability (or unwillingness, etc.) to help you cover these costs is a deal breaker or not.
Money is a finite resource. Your adviser may have very good reasons to attend (his "excuse" certainly doesn't sound "flimsy" to me but very reasonable from the perspective of a PI) and there may simply not be enough money to pay for both of your trips. There may in fact be no money at all to pay for your travel and he may be paying for his travel from some grant that allows him to go to the committee meeting (because the committee is related to the topic of the grant), but that wouldn't allow you to go the conference (because your area of work is not related to the topic of your grant). Since your adviser pays for it (and is legally responsible for the appropriate use of his funds), you need to understand that he gets to make the decision.
As other have pointed out, that does not imply that he doesn't want you to go either. He may wish that he had enough money to send both of you, but doesn't. He may wish that some other (student travel?) money could be found to send you and may in fact be quite happy to write you the necessary letters of recommendation. We could speculate about this all day long, but at the end of the day, you can only elucidate whether this is the case or not by having a conversation.
There may be good reasons why your advisors aren't encouraging you to go to these meetings. For example, they may feel the costs of sending you are out of proportion to the benefits you would get from the experience, particularly given that you are at a very early stage in your PhD. Without knowing more about your specific circumstances, it is difficult to judge.
I suggest you have a friendly, non-confrontational discussion with them about which conferences they think you should aim to attend over the next few years. That way, everyone will know what the goals are, and (hopefully) no confusion will arise in future.