Various polymers are used to produce goods for people, particularly toys for children. To improve the properties of the material, such as plasticity, flame resistance or recyclability, different additives are included into the material composition. However, besides improving their characteristics, these compounds can be significantly harmful to human health: for example, phthalates increase the flexibility of materials and can lead to damage of the internal organs (liver, kidney); polybrominated diphenyl ethers are used as flame retardants and can negatively affect the endocrine system. Thermoplastic rubber is a class of material that combines the characteristics of rubber and plastic and is made from styrene and butadiene. It is characterized by its resistance to weathering and chemicals and its flexibility, and it has a wide range of applications, including in gloves, electronics, toys, footwear, the automotive industry, etc. Thermoplastic rubber children’s toys are popular anti-stress squash toys. However, when they are used, emission of substances is observed, and the toxicity of such toys and their safety for children’s health are unknown. Therefore, the aim of this study was to study the toxicity of popular anti-stress squash toys by biotesting with Lepidium sativum L. to assess the risks to children’s health. The study included a growth test with L. sativum with an assessment of germination energy, seed germination and biometric–morphometric indicators of seedlings. The results were processed using statistical methods. It was found that in all study variants, aqueous extracts from the studied toys or their extracts in a 1% dimethyl sulfoxide solution did not show phytotoxicity. The obtained data indicate the biosafety of water-soluble mixtures secreted by the studied children’s toys.