Thank you for your question. The Directory for the Public Worship of God (DPW) guides us as to Lord's Day observance and worship (you may wish to consult this at http://www.opc.org/BCO/DPW.html).
What has come to be called the Regulative Principle of Worship—that our worship is to be in accordance with Scriptures and, as to elements, is restricted to what the Scriptures teach—serves as a kind of summary of our DPW as well as the Westminster Standards (see, for example, WCF Chapter 21).
While circumstances are not ordered by the Word—where and at what time we should meet, for example—it is hard for me to see how an Easter egg would be a circumstance of worship. This is only to say that it would certainly not be an element of worship (reading and preaching of Scriptures, prayer, the sacraments, etc.); if it is not an element or a circumstance, I do not see the place that such would have in divine worship.
I have no doubt that a spiritual meaning can be assigned to the Easter egg by someone intent on doing such. However, this does not mean that such has any proper part in a service of divine worship on the Lord's Day. I trust that this proves helpful for you.
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