You asked

I'm worried my co-workers may resent me for leaving early to pick up my child.

Make sure that your co-workers know that the hours you get off per week to pick up your child, are purely for the intended purpose. The fact that your boss or manager has given approval for this should also be stated.
To limit feelings of resentment over being allowed to leave work early, make sure that you do the work you missed out on at home. If you can, try to come in earlier in the morning, or at least make sure you are always on time for the start of work. Any assignments you have need to be done within the specified time. Because of perceived favouritism, your work quality will be under more scrutiny than usual from your co-workers. Make certain that the quality of your work is good, or better than normal.
By showing that you are hard working and reliable, any negative feelings from your co-workers should be minimised.

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