"Hereingekrabbelt" is a variation on "Hereinspaziert", which is an informal way of saying, "Come in". (More formal alternatives include "Kommen Sie herein" and "Bitte treten Sie ein".)
The construction is essentially a past participle as a replacement for the imperative ("Ersatzform des Imperativs"). Another example is "Rauchen verboten" ("Smoking prohibited"). Even though this specific example can be interpreted as an elliptic form of "Rauchen ist [hier] verboten", it is clearly an instruction not to smoke.
Other examples of the past participle as a replacement for the imperative were discussed in Why would you use the past participle in commands rather than the imperative?.
The example cited in the question ("Hereingekrabbelt" should be written in one word) has a humorous intent: the building is a toy museum, which attracts many children. Very young children cannot yet walk into the museum ("hereinspazieren", hence "hereinspaziert" from the point of view of the museum), but they can crawl ("krabbeln"); hence "hereingekrabbelt". In other words, the sign says, "Crawl in!" instead of "Come in!".