A bit on copyright - as the photographer, you own the copyright of any shots you take. The subject (or the subject's parents) does not have any right to use those images in any way without your permission, even if they've paid you for the shoot (unless you agree, in writing, to a copyright waiver). There is no legal restriction against non-commercial taking and display of images in a publicly owned place (as opposed to a place that is privately owned but allows public access, eg: a shop), providing those images do not breach decency/obscenity guidelines, and this includes using children as the subject.

But anything that uses people (and in some cases, animals) does require permission for commercial use (note that commercial use doesn't necessarily refer to selling the photograph, but using it as a promotional tool).

If you plan to use the shots in a personal portfolio you need to get a signed model release. If you plan to use those images as part of any promotional material or media advertising - you need a signed release.

In your release form you should state the intended use of the images (eg: website, print collateral, print media, etc), and the estimated lifespan of the public use of the image.

At this stage, it's unlikely that you would need to do that in such detail, but it's worth mentioning to your subjects that you hope to become professional one day and would they mind you using the images as part of your portfolio. If you do decide to use those images down the track you will legally require a signed release.

Here is a link to the photographer's rights / laws website page for commercial use of images - it's a really useful resource and includes links to sample release forms.

HTH!