I see this argument thrown around occasionally, especially by atheist pro-choice people. It's not a serious objection. It's merely meant to trap religious pro-life people. The argument goes like this: If babies who are aborted go to Heaven, then why do you oppose abortion if those babies are going to a better place? I've even seen one atheist pro-life person assert that if she was religious, she wouldn't have any reason to oppose abortion because they went to Heaven. She'd find some other cause to join because abortion wouldn't be that bad. Needless to say, there is absolutely no substance to this argument at all.
It's not absolutely clear in Scripture that children go to Heaven. I believe that they do, because I believe that God is just and I feel it would be unjust to hold someone accountable for their actions if they were incapable of understanding right from wrong. Also, after David's seven-day-old child died, he was confident that he would one day see his child again (see 2 Samuel 12). And we know that Jesus loves children, even to the point of telling us that the kingdom of Heaven belongs to such as them (Matthew 19:14). I think there is good Biblical support that children go to Heaven, but there are those who disagree. The problem is it's just not explicitly spelled out in Scripture, so we can't take a dogmatic position on it. At least not one in which we should feel confident enough in having an abortion because of where we think the child will end up.
God has also commanded us not to murder. This surely includes those who are bound for Heaven. In fact, if this was a good argument, why not take it a step further and say that as soon as someone converts to Christianity, we should kill them to speed their journey along, or to make sure they don't fall away? We are not justified in murdering someone to get them to Heaven.
We are to stand up for those without a voice (Proverbs 31:8). It's the right thing to do. Jesus even said that we have an obligation to be the "good Samaritan," to help those who cannot help themselves, even if other people think that their personal business is more important (Luke 10:25-37).
This is one of those arguments that someone who is seriously invested in the abortion issue wouldn't make. It's as bad as some of the other ones we hear, like "why don't pro-lifers adopt all the unwanted children." Not only is it unsubstantive, but it's also a red herring. Abortion is wrong because an innocent human being is killed. That is reason alone to oppose it.