Max Volume
Master
First decide who you are trying to keep out. An actual thief or your kids. Research what makes a safe less likely to be compromised. Things like metal thickness and whether it is plate or gauge steel and how thick. Research the door and how thick it is and is it laminate (2 pieces of thinner steel) or solid at least 1/4" thick. Also look at edges of the door opening, their thickness and how they are shaped. The door should have very little gap and be somewhat recessed. That makes it more difficult to pry open. Research the locking bolt system and do the bolts just look thick and inside the door are held in place by some chintzy threaded rod or otherwise. Get the best combination of these features you can afford.
If a true thief is determined there will be little to stop them in a short time. As a former mechanic I know there is a tool which fits in a briefcase sized carry case and only needs common type of electrical current to operate and will go through anything like a hot knife through butter. Won't say what to avoid giving anyone ideas. Used one quite a few times on hardened steel pins lengthwise on excavating equipment. Just needed to be careful to not hit the edges and destroy the boom.
If a true thief is determined there will be little to stop them in a short time. As a former mechanic I know there is a tool which fits in a briefcase sized carry case and only needs common type of electrical current to operate and will go through anything like a hot knife through butter. Won't say what to avoid giving anyone ideas. Used one quite a few times on hardened steel pins lengthwise on excavating equipment. Just needed to be careful to not hit the edges and destroy the boom.