D.B.
Yes, I have taken my kids to Makatus Island and it indeed was worth the money and we did stay over 4 hours and had lunch there. My kids are older 4yrs and 10yrs so it is idea for both of them. Your 20 month old would not maybe enjoy it so much and your 3 1/2 old might still be a bit young to take advantage of the huge play area since my 4 yr old had his older brother to follow he felt comfortable. It might be that you wait until they are older and spend the money going to Chuck E Cheese which would definitely cost less and you'll get to eat too.