All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
First of all, we thank you for contacting our site and we thank you and your wife for taking care of those children.
The fact that you are taking care of that girl does not make it permissible for her to show from her body what she is not permitted to show in front of a non-Mahram (marriageable) man. It seems that this girl is completely aware that she is at an age when she is not permitted to appear in front of you in swimming clothes, and this is what our religion orders her to do. This is indeed a praiseworthy act from her. Therefore, you should respect her feelings. Anyway, this does not mean that she prevents herself from enjoying, because she may swim when you are not present in that place. If she needs someone to supervise her when swimming, then your wife may do so.
Moreover, it should be noted that if this girl has a family, then it is more correct to leave her with her family. If she has no family and there is a Muslim family or an Islamic authority that may take care of her, then it is better to leave her with them.
If she is at the age of marriage, she should look for a Muslim man to marry her and one of the Islamic centres will take the responsibility of marrying her off, or she may appoint a Muslim man to marry her off.
Finally, we thank you again for what you are doing.
For more benefit on Islam, please refer to Fataawa 81979, 88906 and 88051.
Allaah Knows best.