We will be traveling to UK, Croatia, Slovenia, and Hungary. Can you help us figure out if there is a SIMM card that will work for a phone in all of these countries. Any help is appreciated.
Many SIM cards will work now in multiple countries without extra cost that the EU has removed roaming charges. If you will start in the UK, you can buy a SIM card there - just make sure it allows roaming at all. If it does - until Brexit, anyway - it should work fine in the other countries you list.
In May, I bought a Dutch Vodafone SIM card on eBay and activated it before leaving the US. (I never even visited the Netherlands.) I used my phone with this SIM card in Slovenia, Italy, and France, and it worked perfectly as soon as I arrived in Europe. Some of the UK SIM cards can be bought ahead of time too though it can be a challenge to top them off without being in the UK (sometimes Paypal will work). But if you are comfortable buying a SIM card in the first country you arrive in, you should be OK for the other countries.