For younger kids, you may want to consider a hydrogen rocket (http://www.ehobbies.com/est1876.html?utm_source=est1876&utm_medium=shopping%2Bengine&utm_campaign=froogle).
Less expensive than hobbie rocket engine replacement, a bit less dangerous (especially if you have younger kids) and fuels some great discussions about power and energy (pun intended:P).
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For younger kids, you may want to consider a hydrogen rocket (http://www.ehobbies.com/est1876.html?utm_source=est1876&utm_medium=shopping%2Bengine&utm_campaign=froogle). Less expensive than hobbie rocket engine replacement, a bit less dangerous (especially if you have younger kids) and fuels some great discussions about power and energy (pun intended :P).