Beyond the proven addictive effects of handing a dopamine device to your kid, there are legal ramifications many parents aren't aware of.
WhatsApp and TikTok aren't just there like air, free for all to consume. They are service providers and both sides are bound by a contract, the EULA. IIRC, WhatsApp recently reduced it minimum age from 16 to 12. So if you install WhatsApp on your 8 year old's phone, you have broken the contract.